RFC: rawget Type Operator (#39)

* Initial commit for rawget

* rfc for rawget type operator completed

* Forgot .md

* Code blocks '''lua -> '''luau

* grammar fixes

Co-authored-by: vegorov-rbx <75688451+vegorov-rbx@users.noreply.github.com>

* Revised motivation section and design section

* Revised the drawbacks section

* Added a note about how this type operator doesn't work with class types

* Rephrased the summary

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* Redirect users to open-source Luau global functions

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* reword "runtime pair" to counterpart and update the link

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* grammar fix: "interactions" -> "interaction"

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* Revised wording for motivation section

Co-authored-by: aaron <aweiss@hey.com>

* Added an article and broke up a sentence into two

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* Revided the reasoning for second drawback

* Update docs/rawget-type-operator.md

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Co-authored-by: vegorov-rbx <75688451+vegorov-rbx@users.noreply.github.com>
Co-authored-by: aaron <aweiss@hey.com>

docs/rawget-type-operator.md

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+# `rawget` type operator
+
+## Summary
+
+This RFC proposes the addition of a new type operator, `rawget`, which can be used to look up a specific property of a table type *without* invoking the `__index` metamethod.
+
+## Motivation
+
+Currently, `rawget` is a built-in runtime operator in the language ([rawget in Luau Global functions](https://luau-lang.org/library#global-functions)) that is missing a corresponding type operator that captures its behavior. This RFC seeks to address this hole in the type system by providing a builtin type operator for `rawget` that will allow Luau developers to express more accurate types in their code:
+
+```luau
+local prop: rawget<typeof(someExpr), "someProp"> = rawget(someExpr, "someProp")
+```
+
+## Design
+
+The functionality of the `rawget` type operator behaves the same as its [runtime counterpart](https://luau-lang.org/library#global-functions). Namely, it provide a way to look up a specific property of a table type without invoking the `__index` metamethod. 
+ 
+```luau
+local var1 = {
+  property = "Hello"
+}
+
+local var2 = setmetatable({ }, { __index = var1 })
+type doesntExist = rawget<typeof(var2), "property"> -- reduces to an error
+
+local var3 = setmetatable({ property = 1 }, { __index = var1 })
+type doesExist = rawget<typeof(var3), "property"> -- doesExist = number
+```
+
+Error messages would be consistent with those of the `index` type operator:
+```luau
+type doesntExist = rawget<typeof(var2), "property">  -- Error message: Property '"property"' does not exist on type 'var2'
+
+local key = "property"
+type age = rawget<Person, key> -- Error message: Second argument to rawget<Person, _> is not a valid index type; Unknown type 'key'
+```
+
+Note: `rawget` type operator does not work on class types because they do not have direct fields that can be looked up without invoking its metamethod.
+
+The implementation effort for this type operator is very minimal. Since the `rawget` type operator functions similarly to the `index` type operator, we can reuse the functions already used to implement the `index` type operator.
+
+## Drawbacks
+
+There aren't appreciable drawbacks. One possible drawback is the increase in the size of the codebase, which may complicate maintenance. However, this is not expected to be a major drawback since the code for `index` type operators already exists. By reusing the code, it is even fair to say that there will be less than 50 lines of code needed to implement this type operator.
+
+Another drawback can come from the extra knowledge a user may need in order to use this type operator. For example, users will need to know the inner workings of types and how different operations interact with them (for instance, how `index` interacts with metatables that have the `__index` metamethod). However, this additional complexity is manageable because the language already incorporates such complexities at the runtime level. Developers are already familiar with concepts like `rawget`, so this does not add much new complexity. 
+
+## Alternatives
+
+An alternative to the `rawget` type operator is to enable users to modify the behavior of the existing `index` type operator to control its recursive nature. While this approach offers flexibility, it requires additional design considerations and complicates both usage and implementation. However, the introduction of "user-defined type functions" is planned for the near future. This will allow users to create their own type operators, making this alternative design feasible for developers to implement.