fix(react): apply CommonMark flanking rules to composer emphasis

[ishan-one8]

Apply CommonMark flanking rules to composer emphasis

Root cause. appendMarkdown in packages/react/src/lib/contentEditableComposer.js treated any _, *, __ or ** as an emphasis delimiter and paired it with the next identical marker, without ever checking whether that delimiter run was allowed to open or close emphasis. CommonMark decides that with the left/right-flanking rules and additionally forbids intra-word _, so any underscore or asterisk with another one later in the line opened emphasis.

Fixes #1372

Acceptance Criteria fulfillment

• my_file_name.js, call foo_bar_baz() now and 2 * 3 * 4 = 24 render literally again
• **bold**, *italic*, __init__, a _b_ c and intra-word foo*bar*baz still render as emphasis
• Ships with a test that fails before the fix and passes after
• Inline code, strikethrough, links and <br> paths untouched
• yarn build still produces dist/cjs and dist/esm (exit 0)

Video/Screenshots

Not applicable — no visible UI change beyond the corrected text rendering, which the tests cover.

PR Test Details

Two commits:

1. test(react): emit CommonJS under the test env so jest can run — babel.config.js had modules: false unconditionally, so babel-jest handed ESM to jest and every test file died with "Cannot use import statement outside a module". It now emits CommonJS only when api.env('test') is true; the rollup build still gets native ES modules, which is why the bundle output is unchanged. Also adds the missing test script. This is the same area as fix(react): src/index.test.js fails on develop due to missing swiper/element/bundle module #1191.
2. fix(react): apply CommonMark flanking rules to composer emphasis — adds canOpenEmphasis / canCloseEmphasis / findClosingMarker and uses them when selecting a delimiter. When the nearest matching marker cannot close, the scan continues to the next one instead of giving up.

Before the fix, the new test fails:

✕ does not treat underscores inside a word as emphasis
✕ does not open emphasis on a marker followed by whitespace
    Expected: "2 * 3 * 4 = 24"
    Received: "2 <em> 3 </em> 4 = 24"

After it, yarn test in packages/react passes 3/3.

I also diffed the fixed renderer against the reference commonmark package across 13 emphasis cases — including foo*bar*baz, snake_case and _real_ emphasis, _leading, trailing_ and 5 * 6 — and all 13 now match.