Lexer: add offset getter (#1688)

Added a getter for the Lexer's private `offset` to track its cursor
position in the buffer.
This helps me index tokens by buffer address in a project where I'm
rendering Luau code with [Dear ImGui](https://github.com/ocornut/imgui).
Would love this merged so I can use official Luau releases again!

Co-authored-by: Adrian Duermael <adrien@cu.bzh>

Ast/include/Luau/Lexer.h

@@ -187,6 +187,11 @@ public:
     static bool fixupQuotedString(std::string& data);
     static void fixupMultilineString(std::string& data);
 
+    unsigned int getOffset() const
+    {
+        return offset;
+    }
+
 private:
     char peekch() const;
     char peekch(unsigned int lookahead) const;