feat: support ==text== highlighting in the GFM flavour

[jetbrains-air]

Fixes IJPL-175082: ==...== syntax is not rendered as highlighted, while being an official extended-syntax alternative to the HTML <mark> tag.

What

Adds ==text== highlighting to GFMFlavourDescriptor, following the same pattern previously used for strikethrough and math:

• Lexer: = is now lexed as a separate token (GFMTokenTypes.EQUALS) instead of being merged into TEXT, and \= was added to the backslash-escape rule (CommonMark treats every ASCII punctuation character as escapable). _GFMLexer.kt is regenerated from gfm.flex with the IntelliJ JFlex 1.9.2 fork and idea-flex.skeleton; the regeneration toolchain was validated by first reproducing the committed lexer byte-for-byte from the unmodified gfm.flex.
• Parser: a new HighlightDelimiterParser (mirroring StrikeThroughDelimiterParser) participates in EmphasisLikeParser and produces a GFMElementTypes.HIGHLIGHT node. Only runs of exactly two = count, so =text= and ===text=== stay plain text, matching markdown-it-mark/Obsidian behavior.
• HTML: HIGHLIGHT renders as <mark> via the existing EqualDelimiterTrimmingInlineTagProvider.

Like the earlier math support, the token is added to the GFM lexer only; SFMFlavourDescriptor inherits the parser but its lexer emits no EQUALS tokens, so its behavior is unchanged.

AST note

= characters in inline text are now separate = tokens instead of being merged into adjacent TEXT tokens (the same kind of change ~ and $ introduced). Two existing parser goldens (math.txt, gfmTableWithBlockQuote.txt) were regenerated accordingly; rendered HTML is unaffected.

Testing

• New parser AST test (testHighlight in MarkdownParsingTest) and HTML test (testHighlight in HtmlGeneratorCommonTest) covering: basic and multi-word highlights, single/triple/quadruple =, intraword, adjacent pairs, backslash escapes, unclosed delimiters, = in plain text and URLs, nesting with strong/strikethrough/links/code spans, and a setext heading regression case.
• Full JVM suite: 1551 tests pass (includes CommonMark and GFM spec suites). JS suite passes; JS/Native/WASM targets compile.

Produced by Air Automations. Name: Markdown library: fix the bug / Run: https://air.jetbrains.cloud/org/05cf1a7f-6ab5-713b-abd3-29d0c8a05e2d/automations/cb6a17f6-e7a9-41a8-9611-7c95c4eb0141?run=faef9ac2-8bf0-4db1-8e17-4dce310f599f