[lexical-markdown] Bug Fix: replace lookbehind assertions for Safari < 16.4 compatibility

[Yahid-Basha]

Description

Current behavior: Negative lookbehind assertions ((?<!...)) inside
createTextFormatTransformersIndex throw a SyntaxError at new RegExp(...)
construction time in Safari < 16.4 (ES2018 not supported), crashing markdown
import entirely for those users.

Changes: Replace all lookbehind-based patterns with a preceding-character
capture group (^|[^\\...]) that matches start-of-string or a non-escape
character before the delimiter. Call sites offset match.index by
match[1].length to find the real start of the span. The escape guard on
openTagsRegExp is removed since isEscaped() in scanDelimiters() already
handles this.

Closes #8012

Test plan

Before

Safari < 16.4 throws SyntaxError: Invalid regular expression: invalid group specifier name when any markdown string is imported — no markdown rendering
works at all.

After

7 new unit tests added to LexicalMarkdown.test.ts:

• No throw on regex construction
• Code span at start of string
• Code span mid-text
• Multiple spans on one line
• Escaped backtick not parsed as span
• Content capture correctness
• No emphasis formatting inside code span

All 2,754 existing tests continue to pass.

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Project	Deployment	Actions	Updated (UTC)
lexical	Ready	Preview, Comment	Apr 11, 2026 5:49am
lexical-playground	Ready	Preview, Comment	Apr 11, 2026 5:49am