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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