Reserve the dollar sign token (#44)

docs/reserve-dollar-sign.md

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+# Reserve dollar sign (`$`) to be forever unused
+
+## Summary
+
+This RFC proposes a stable contract that `$` will not be used for any features in Luau, and will not be accepted as part of identifier tokens.
+
+## Motivation
+
+By reserving `$` external pattern matching tools, which at time of writing are already planned to be created by both Roblox and full-moon, can use it to symbolize things like placeholder tokens, similar to [ast-grep](https://ast-grep.github.io/guide/pattern-syntax.html). 
+
+For example, a search in ast-grep for "checking equality to itself" would be `$A == $A`. This becomes ambiguous and harder to write if `$A` is also a valid identifier, or means some potential expression.
+
+## Design
+
+Nothing more than a stable contract is necessary. `$` will not be allowed in identifiers, and `$` will not be a component of any syntax being added to the language.
+
+`$` is chosen as it seems fairly unlikely we will ever use it for any syntax, while at the same time its use as a placeholder token is common.
+- ast-grep, as mentioned before
+- SQL uses `$` for binded parameters in prepared statements
+
+## Drawbacks
+
+The dollar sign is used in a couple of places outside Luau:
+
+- JavaScript, C++, and probably others allow it as an identifier, which jQuery uses as its primary controller
+- PHP, Perl, Bash, and others both use it for variables (`$name = "John"`)
+- Haskell uses it as an infix operator as sugar to help clean up function applications
+
+These use cases are nowhere near compelling enough to where we would likely do the same.
+
+## Alternatives
+
+- Don't do it, as always.
+- Pick a different sigil, though `$` is a fairly clear choice