Refactor: Restructure extensions and Unicode modules following module philosophy

[ocots]

This PR restructures the codebase to follow the module philosophy outlined in dev/philosophy/PHILOSOPHY.md.

Changes

Extension Structure

• Moved extensions from ext/*.jl to ext/ExtensionName/ subdirectories with proper entry_point.jl pattern
• Moved extension implementations to src/Extensions/ submodules:
  • coverage_postprocessing.jl
  • documenter_reference.jl
  • test_runner.jl

Unicode Module

• Split Unicode module into submodules:
  • src/Unicode/subscripts.jl
  • src/Unicode/superscripts.jl

Qualified Imports

• Updated all imports to use qualified paths:
  • CTBase.Extensions.run_tests (was CTBase.run_tests)
  • Unicode.ctindices, Unicode.ctupperscripts, etc.
• Updated documentation to reflect new qualified import paths

Test Organization

• Restructured test files to follow new module organization
• Removed redundant test files:
  • test/suite/unicode/test_utils.jl (consolidated into test_unicode_enriched.jl)
  • test/suite/exceptions/test_display.jl (consolidated into test_exception_display.jl)

Documentation Updates

• Updated BREAKINGS.md, CHANGELOGS.md with new qualified paths
• Updated docs/src/guide/test-runner.md and docs/src/index.md

All changes maintain backward compatibility while enforcing the qualified import convention.

[ocots]

This PR implements the module philosophy refactoring outlined in dev/philosophy/PHILOSOPHY.md. The main changes are:

1. Extension structure: Moved from flat ext/*.jl files to proper ext/ExtensionName/ subdirectories with entry_point.jl pattern, following the established convention for weak dependencies.

2. Implementation relocation: Extension implementations moved from ext/ to src/Extensions/ submodules (coverage_postprocessing.jl, documenter_reference.jl, test_runner.jl) to keep source code in src/.

3. Unicode module split: Split the Unicode module into subscripts.jl and superscripts.jl submodules for better organization.

4. Qualified imports enforcement: Updated all imports to use qualified paths (e.g., CTBase.Extensions.run_tests instead of CTBase.run_tests, Unicode.ctindices instead of CTBase.ctindices). This enforces the "no top-level exports" convention.

5. Documentation updates: Updated BREAKINGS.md, CHANGELOGS.md, and all guide documentation to reflect the new qualified import paths.

6. Test consolidation: Removed redundant test files and consolidated tests to follow the new module organization.

All changes maintain backward compatibility while strictly following the module philosophy conventions.