Add TEXTJOIN function

[marcin-kordas-hoc]

Summary

• Implements TEXTJOIN(delimiter, ignore_empty, text1, [text2, …]) in TextPlugin
• Supports scalar and array/range delimiters (with cycling when delimiter is a range)
• Skips empty strings when ignore_empty is TRUE; includes them when FALSE
• Returns #VALUE! when the result exceeds 32,767 characters (Excel cell limit)
• Adds ErrorMessage.TextJoinResultTooLong for the length-exceeded error
• Adds translations for all 17 supported languages
• Documents the function in docs/guide/built-in-functions.md

Test plan

• 22 unit tests in test/textjoin.spec.ts covering basic usage, ignore_empty behaviour, array/range delimiters with cycling, type coercion, error propagation, and edge cases
• Smoke tests in test/smoke.spec.ts include TEXTJOIN scenarios
• All 41 tests pass (npm run test:jest)
• TypeScript compiles without errors (npm run compile)
• No lint errors (npm run lint)

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Note

Medium Risk
Adds a new built-in formula function in the interpreter, including range flattening/coercion and a hard output-length limit, which could affect evaluation behavior and error propagation in text-heavy sheets. Changes are localized but touch core formula execution paths and add broad i18n updates.

Overview
Adds the TEXTJOIN(delimiter, ignore_empty, text1, ...) built-in function, supporting scalar or range/array delimiters (cycled between joined values), optional skipping of empty strings, and a new #VALUE! error when the output exceeds 32,767 characters via ErrorMessage.TextJoinResultTooLong.

Updates function-name translations across all supported languages, documents TEXTJOIN in built-in-functions.md, and adds comprehensive coverage via new textjoin.spec.ts plus additional smoke tests.

Docs demos are switched from embedded CodeSandbox iframes to Stackblitz links, and package-lock.json is updated (notably adding platform-specific esbuild optional packages).

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