fix: adjust scope created within multiline string W-21968889

[peternhale]

Summary

Refines multiline string ('''…''') highlighting and template interpolation for single-quoted strings only, with tests and regenerated grammars (Apex + SOQL + Atom cson).

Multiline strings (multiline-string-literal)

• Begin: '''(?=$) so the opening ''' must sit at end of line (matches the intended “newline after opener” behavior under line-based tokenization).
• Scopes: compound string.quoted.single.apex + string.quoted.single.multiline.apex so themes color MLS body like normal string text.
• Body: only #string-character-escape — #string-template-expression is not included for MLS, so ${…} is not meta.template-expression; it uses the same string scopes as the rest of the MLS. Only backslash escapes get constant.character.escape.apex.

Simple strings (string-literal — unchanged structure)

• Template begin: (?<!\\)\$\{ so \${…} is literal (escaped $), not a template opener.
• Template interior: \G-anchored match supporting dotted names (e.g. ${a.b}) without splitting on ..

Other

• when-string: include #multiline-string-literal before #string-literal in the relevant capture groups so MLS is available alongside single-quoted strings where those patterns are used.

Files

• src/apex.tmLanguage.yml — grammar rules above.
• grammars/apex.tmLanguage, grammars/apex.tmLanguage.cson, grammars/soql.tmLanguage — rebuilt outputs.
• test/literals.test.ts — expectations for MLS (no template tokens inside '''…'''), escaped \$, dotted ${a.b} in simple strings, and updated MLS fixtures for EOL ''' opener.

Verify

• npm run build && npm test

@W-21968889@