Use execa over spawn

[lopert]

Problem

The test helpers fail on Windows because child_process.spawn() cannot resolve .cmd files (which npm uses for executables on Windows). This causes the library to fail when trying to execute the Shopify CLI on Windows systems. (#46)

Solution

Shopify CLI also has cross-platform requirements, so I looked at that repo for a solution. Instead of using spawn, it instead uses execa.
This PR migrates us from child_process.spawn to execa (version 7.2.0, matching Shopify CLI).

Changes

Implementation

• Added dependency: execa@^7.2.0 to package.json
• Updated build-function.ts: Replaced spawn with execa, maintained exact error message compatibility
• Updated get-function-info.ts: Replaced spawn with execa, maintained exact error message compatibility

Tests

• Updated test mocks: Changed from EventEmitter-based spawn mocks to simple promise-based execa mocks
• Maintained behavior: All error messages and throwing behavior remain identical for backward compatibility

Tophat 🎩

• Clone the repo, checkout this branch.
• pnpm install
• pnpm pack -> should give you a tarball.
• In a standalone app/function, update the function's devDependency to point at the tarball. Something like:

  "devDependencies": {
    "@shopify/shopify-function-test-helpers": "file:/Users/lopert/src/github.com/Shopify/shopify-function-test-helpers/shopify-shopify-function-test-helpers-1.0.0.tgz",
    "vitest": "^3.2.4"
  }

• confirm the code that was pulled into the node_modules of the app/function contains the updated execa using code.
• run npx vitest run

Alternative

We could pass the shell arg to spawn when on windows, #47

[saga-dasgupta]

LGTM, thanks for fixing this.