feat(editor): add indentation folding for legacy modes(as fallback)

[bajrangCoder]

No description provided.

[greptile-apps]

Greptile Summary

Adds indentation-based code folding as a fallback for plain text and legacy StreamLanguage modes.

• Registers the fallback alongside CodeMirror’s fold gutter when code folding is enabled.
• Computes fold ranges from contiguous, more-deeply-indented nonblank lines while preserving internal blank lines.
• Adds coverage for indentation folding in a legacy stream language.

Confidence Score: 5/5

The pull request appears safe to merge, with no concrete correctness or security defects identified.

The fallback is limited to plain text and legacy stream languages, respects the existing code-folding toggle, and leaves syntax-aware folding unchanged for modern language modes.

Important Files Changed

Filename	Overview
src/cm/baseExtensions.ts	Registers the indentation fold service only when the existing code-folding option is enabled.
src/cm/indentationFolding.ts	Implements cached indentation fold discovery for plain text and StreamLanguage modes while deferring modern language modes to syntax-aware folding.
src/test/editor.tests.js	Verifies that legacy stream modes fold an indented block across an internal blank line.

_{Reviews (1): Last reviewed commit: "feat(editor): add indentation folding fo..." | Re-trigger Greptile}