Fix parsing of generic type parameters in arrow functions with JSX syntax

[poteboy]

Potentially fixes #15713

This PR addresses an issue where the TypeScript parser incorrectly interprets generic type parameters in arrow functions as JSX elements. Specifically, the code const a = <T>(b: T) => b was being parsed as a JSX element instead of a generic type parameter.

Testing:

Added a new test case jsxGenericArrowFunction.tsx to verify the fix. Before the change, the following errors were observed:

jsxGenericArrowFunction.tsx(1,12): error TS2304: Cannot find name 'T'.
jsxGenericArrowFunction.tsx(1,12): error TS17008: JSX element 'T' has no corresponding closing tag.
jsxGenericArrowFunction.tsx(1,22): error TS1382: Unexpected token. Did you mean `{'>'}` or `>`?
jsxGenericArrowFunction.tsx(1,25): error TS1005: '</' expected.

After the change, these errors no longer occur.

This change ensures that generic type parameters in arrow functions are correctly parsed, improving TypeScript's type-checking and syntax parsing capabilities.

Additionally, as noted in the Design Meeting Notes, there has been anticipation for a fix to this issue from the TypeScript community

[poteboy]

@microsoft-github-policy-service agree

[DanielRosenwasser]

This would technically be a breaking change for if someone wrote something like

const a = <Formatted>() => 1234</Formatted>;

I'm surprised that we don't have a test like that at all though.

[poteboy]

@DanielRosenwasser
You're right, I didn't take this case into account until the CI failed. Parsing this case as an arrow function and backtracking when we hit the JSX closing element would solve the issue, but it could potentially have exponential time complexity. If that's acceptable, it could be a viable solution.