parserPluginsForPath enabled Babel's typescript plugin for .js, .jsx,
.mjs and .cjs sources. TypeScript reads some valid JavaScript
differently: a conditional whose consequent is an arrow with a
parenthesized body (c?x=>(a):y=>(b)) throws as a return-type
annotation, and a<b>(c) becomes a generic call. VS Code's 20 MB
workbench bundles therefore returned no AST at all.
Select JavaScript grammar (JSX plus legacy decorators) for explicit
JavaScript extensions. TypeScript dialects and unnamed sources keep the
existing TypeScript-capable contract.
Fixes#3780
Refs #3670
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>