Use hermes-parser via babel-plugin-syntax-hermes-parser for Flow files

[jonreading81]

Fixes #80.

Summary

The previous parse step used @babel/parser with the flow plugin, which lags behind Flow's own parser and rejects modern syntax (readonly, component, hook, renders, match, keyof, T[K], x is T, <T extends U>, etc). The extension errored out on any file using those constructs and no marker appeared.

This PR switches to a single transformSync() with babel-plugin-syntax-hermes-parser in the plugin list. Its parserOverride hook swaps in Hermes (Flow's canonical parser) for .js / .jsx / .mjs files, and is a no-op for .ts / .tsx, so TypeScript continues to be parsed by Babel's built-in parser (configured via parserOpts.plugins). Same output shape, same logger contract — only the parser changes.

Verified locally

• Modern Flow syntax: type Props = { ref?: React.RefObject<{ readonly closeMenu: () => void }> } — now compiles cleanly (was throwing at parse time).
• TypeScript: React.FC<Props> with type annotations — continues to compile without regression.
• Legit bailout (ref access during render) — still emits the correct "Cannot access refs during render" diagnostic.

Alternatives considered

The original suggestion in #80 was a separate SWC Flow-strip pass. Your feedback pointed at babel-plugin-syntax-hermes-parser as cleaner — same Babel run, no SWC dependency, uses Flow's canonical parser. I agree and went with that.

Notes

• Added babel-plugin-syntax-hermes-parser@^0.36.1 to packages/server/dependencies.
• Removed the intermediate BabelParser.parse() step — now a single transformSync(). @babel/parser remains a dep but is unused in the server source after this change; left in to keep the diff minimal.