feat(lexer): emit one jsx_text token per JSX text child in .jsx (#61)

[ericsssan]

Addresses #61 for plain JSX (.jsx). TSX coverage is a documented follow-up (see below).

Problem

JSX child text was tokenized as ordinary JS tokens and the surrounding whitespace was dropped:

<div>
   unrelated{ foo }
</div>

produced identifier "unrelated" with the leading "\n " missing — diverging from espree/@typescript-eslint, which emit one JSXText token per text node. The jsx_text tag and jsx_text_node AST node already existed with the right range; only the token stream was wrong.

Fix

The lexer is context-free, so it now tracks JSX structure itself: a stack of frames, each an element BODY (text context) or an expression container EXPR. <tag> pushes a BODY, </tag> pops it; { in a body pushes EXPR and the matching } pops it (the frame's low bits count nested object/block braces so the right } closes the container); self-closing <br/> and fragments <> are handled. In body context the scanner emits everything up to the next < { > } as one jsx_text token, whitespace included. A bare >/} ends the run and is lexed as its own token, which the parser rejects (matching the reference).

The issue repro now yields exactly JSXText[5,18] ("\n unrelated") + JSXText[37,38].

Scope: .jsx only (not .tsx)

In TSX a <T> is ambiguous — JSX element vs generic type-args vs generic arrow — and only the parser's speculative parse can disambiguate. Committing to "this is JSX" in the lexer regressed 14 tsx/ts conformance cases (<T,>() =>, <Foo<T>>, …). So element-body tracking is gated off for TS (jsx_text_mode = is_jsx and !is_ts); TSX keeps its prior token stream. Full .tsx coverage needs the parser-driven / post-lex approach — filed as a follow-up.

Performance

jsx_text_mode is a comptime parameter, so the entire text-tracking path compiles away for non-JSX input — the hot lexer used by the overwhelming majority of files is byte-for-byte identical to before. Measured on a 274 KB non-JSX file: no perf delta (interleaved runs overlap, best-of 3.65s vs 3.63s).

Validation

• New token-level tests: the repro, multi-word text (one token), nested elements, expression containers, fragments, self-closing; plus gating tests that TSX and plain JS emit no jsx_text.
• Full suite green incl. test262 3966/3966 · 1389/1389; babel 1928/1928 · 1548/1548; TS 17910/17913 · 1210/1223 — all at baseline.