Fix TEXTJOIN PR: remove tests from public repo, fix docs ordering

[marcin-kordas-hoc]

Context

Addresses review feedback from @sequba on PR #1625 (Add TEXTJOIN function).

sequba pointed out the test placement issue twice:

1. March 2 — "Do not add tests to [smoke.spec.ts]. We want to keep it very basic."
2. March 13 — "Please, move tests to hyperformula-test repository"

This branch contains the full TEXTJOIN implementation from feature/TEXTJOIN_w_docs with the following corrections applied.

Changes

• Removed test/textjoin.spec.ts — tests belong in the private hyperformula-tests repository
• Removed TEXTJOIN tests from test/smoke.spec.ts — restored smoke tests to their original minimal state
• Fixed alphabetical ordering in docs/guide/built-in-functions.md — moved TEXTJOIN entry from between SUBSTITUTE/T to the correct position between TEXT/TRIM
• Added test/fetch-tests.sh and test:setup-private npm script (synced from develop) for cloning the private test repo
• Added /test/hyperformula-tests/ to .gitignore
• Prepared test/hyperformula-tests/textjoin.spec.ts (29 tests, all passing) — needs to be pushed to the hyperformula-tests private repo

Implementation review

The TEXTJOIN implementation was reviewed against other function patterns in the codebase:

• Function metadata (implementedFunctions) uses repeatLastArgs: 1 with FunctionArgumentType.ANY for delimiter and text args — correct pattern for variable-arg functions that handle ranges manually
• flattenArgToStrings helper follows the same range iteration pattern as SUMPRODUCT, DateTimePlugin, and NumericAggregationPlugin (arg instanceof SimpleRangeValue ? arg.valuesFromTopLeftCorner() : [arg])
• Empty value handling via coerceScalarToString (EmptyValue → '') with subsequent filtering is correct
• Error propagation, delimiter cycling, and 32,767 character limit all work correctly
• All 17 i18n translations are present and correct

How did you test your changes?

• npm run compile — TypeScript compiles without errors
• npm run lint — 0 errors (only pre-existing warnings)
• npx jest test/smoke.spec.ts — all 4 smoke tests pass
• npx jest test/hyperformula-tests/textjoin.spec.ts — all 29 TEXTJOIN tests pass (locally)

Types of changes

• New feature or improvement (a non-breaking change that adds functionality)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. Fixes Add TEXTJOIN function #1625

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.