test: skip yarn package manager test if not present

[suveshmoza]

Overview

• Skips yarn package manager test when yarn is not present.
• Added isYarnInstalled util to check yarn presence.

Manual Testing

1. Without yarn installed, run bun run --filter wxt test yarn

wxt test $ buildc --deps-only -- vitest
│ [5 lines elided]
│ 
│  RUN  v4.1.5 /Users/suveshmoza/oss/wxt/packages/wxt
│ 
│  ↓ src/core/package-managers/__tests__/yarn.test.ts (2 tests | 2 skipped)
│ 
│  Test Files  1 skipped (1)
│       Tests  2 skipped (2)
│    Start at  15:49:37
│    Duration  118ms (transform 10ms, setup 10ms, import 13ms, tests 0ms, environment 0ms)
└─ Done in 470 ms

2. With yarn installed, run bun run --filter wxt test yarn

wxt test $ buildc --deps-only -- vitest
│ [5 lines elided]
│ 
│  RUN  v4.1.5 /Users/suveshmoza/oss/wxt/packages/wxt
│ 
│  ✓ src/core/package-managers/__tests__/yarn.test.ts (2 tests) 246ms
│ 
│  Test Files  1 passed (1)
│       Tests  2 passed (2)
│    Start at  15:50:48
│    Duration  852ms (transform 10ms, setup 10ms, import 501ms, tests 246ms, environment 0ms)
└─ Done in 1.16 s

This PR closes #2371

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.60%. Comparing base (7905cb3) to head (8000051).

Files with missing lines	Patch %	Lines
packages/wxt/src/core/package-managers/yarn.ts	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2397      +/-   ##
==========================================
+ Coverage   78.46%   78.60%   +0.13%     
==========================================
  Files         133      133              
  Lines        3882     3888       +6     
  Branches      875      876       +1     
==========================================
+ Hits         3046     3056      +10     
+ Misses        749      746       -3     
+ Partials       87       86       -1     

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commit: 8000051