Feat: Add structured text highlighting support.

[mm-kpk]

Implementing feature request #2936.

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Summary by cubic

Add syntax highlighting for IEC 61131-3 Structured Text in code snippets. Registers the iecst language with highlight.js and adds types for smooth TS support.

• New Features

  • Register iecst from highlightjs-structured-text in CodeSnippet so language="iecst" is highlighted.
  • Add a d.ts module declaration for highlightjs-structured-text.

• Dependencies

  • Add highlightjs-structured-text@^1.7.3 (brings [email protected] as a sub-dependency via lockfile).

^{Written for commit cffe15a. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

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