Support ["prop"] syntax on class definitions (#704)

Some classes have properties which are not valid identifiers (such as
https://create.roblox.com/docs/reference/engine/classes/Studio)

This adds support for the following syntax in definition files:
```lua
declare class Foo
    ["a property"]: string
end
```

Closes #702

Ast/src/Parser.cpp

@@ -905,6 +905,25 @@ AstStat* Parser::parseDeclaration(const Location& start)
             {
                 props.push_back(parseDeclaredClassMethod());
             }
+            else if (lexer.current().type == '[')
+            {
+                const Lexeme begin = lexer.current();
+                nextLexeme(); // [
+
+                std::optional<AstArray<char>> chars = parseCharArray();
+
+                expectMatchAndConsume(']', begin);
+                expectAndConsume(':', "property type annotation");
+                AstType* type = parseTypeAnnotation();
+
+                // TODO: since AstName conains a char*, it can't contain null
+                bool containsNull = chars && (strnlen(chars->data, chars->size) < chars->size);
+
+                if (chars && !containsNull)
+                    props.push_back(AstDeclaredClassProp{AstName(chars->data), type, false});
+                else
+                    report(begin.location, "String literal contains malformed escape sequence");
+            }
             else
             {
                 Name propName = parseName("property name");

tests/TypeInfer.definitions.test.cpp

@@ -362,4 +362,21 @@ TEST_CASE_FIXTURE(Fixture, "class_definition_overload_metamethods")
     CHECK_EQ(toString(requireType("shouldBeVector")), "Vector3");
 }
 
+TEST_CASE_FIXTURE(Fixture, "class_definition_string_props")
+{
+    loadDefinition(R"(
+        declare class Foo
+            ["a property"]: string
+        end
+    )");
+
+    CheckResult result = check(R"(
+        local x: Foo
+        local y = x["a property"]
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ(toString(requireType("y")), "string");
+}
+
 TEST_SUITE_END();