Add optional previewPath to file previewer

[tennashi]

This PR introduces an optional previewPath property in Detail, which allows specifying a custom preview path for files.
If the previewPath is provided, it will be used for previewing the file.
Otherwise, the path will be used as a fallback.

This change allows for scenarios where an item’s path is a directory. For example, when the path points to a directory, it enables the previewing of a specific file like README.md inside that directory.

This update is backward compatible, so it won't affect existing functionality.

If there are existing features that can achieve the same functionality, please let me know.

The test coverage check is failing, but there are no similar tests for this change in the builtin extension, so I have left it on hold for now.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 51.46%. Comparing base (88d3c5a) to head (3f69a65).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
builtin/previewer/file.ts	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   51.48%   51.46%   -0.03%     
==========================================
  Files          78       78              
  Lines        2317     2318       +1     
  Branches       70       70              
==========================================
  Hits         1193     1193              
- Misses       1124     1125       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[lambdalisue]

Please use previewPath instead