Fix JSX EOF tag callback crash

[SebTardif]

Summary

• fix the JSX opening-tag callback so incomplete tags at end of input do not throw a RangeError
• add a regression test that exercises a real JSX context with const x = <Foo in JavaScript and TypeScript
• keep the fix scoped to the shared callback used by both grammars

Verification

• /home/sebtardif/flutter/bin/flutter test test/api/highlight_test.dart
• demonstrated the new regression test fails against the pre-fix callback implementation

Bug appears to originate from commit c8288a4 (first commit, 2024-02-03).