feat: add status bar item

[tjkandala]

Use the new Status Bar API to show file coverage ratios and link to commit reports. As a result, the Codecov extension now supports the Sourcegraph commit page.

Notes:

• Debounce setStatusBars and updateContext calls to prevent doing unnecessary work on diff views.
• I initially tried factoring out API calls shared between setStatusBars and updateContext, but I decided against it since the result is memoized anyways.

TODO:

• Update extension-api-stubs after feat: support extension API v25.2.0 sourcegraph/extension-api-stubs#137 is merged.
• Test the extension on Sourcegraph 3.25

Has been published to sourcegraph.com for testing

[codecov]

Codecov Report

Merging #68 (d79004a) into master (b29c80c) will decrease coverage by 0.71%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master     #68      +/-   ##
=========================================
- Coverage    9.78%   9.06%   -0.72%     
=========================================
  Files           8       8              
  Lines         378     408      +30     
  Branches       99     107       +8     
=========================================
  Hits           37      37              
- Misses        341     371      +30     

Impacted Files	Coverage Δ
src/extension.ts	0.00% <0.00%> (ø)

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