feat: implement runtime package manager detection

[Eazash]

This PR adds a detectRuntimePackageManager api function to detect the current package manager in runtime using the npm-config-user-agent environment variable. It also adds a refactor so that argv parsing can be a safe fallback in the event that the environment variable is not set.

The possible usecases for this API include

• Allowing scripts that contain a package manager selector to default to and/or suggest the package manage currently in use
• Enabling silent scripts that don't require user input for package installation (for use in CI environments)

I'm currently marking this as a draft because I'd appreciate suggestions on how to implement tests for this new API.

References

• pnpm/pnpm#4317
• npm/cli#1919
• yarnpkg/yarn#7127

[pi0]

This is nice idea!

• Can you please add tests to make sure pnpm and yarn (v1 and berry+) are detected? (we can use mocking of process to avoid extra exec overhead)
• We shall also implement logic for arv0 (runtime binary as package manager: bun/deno) -- we might use join + prefix check

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 18.00000% with 41 lines in your changes missing coverage. Please review.

Project coverage is 60.00%. Comparing base (660392f) to head (a7e121f). Report is 67 commits behind head on main.

Files with missing lines	Patch %	Lines
src/package-manager.ts	27.27%	24 Missing :warning:
src/cli.ts	0.00%	17 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #164       +/-   ##
===========================================
- Coverage   82.17%   60.00%   -22.18%     
===========================================
  Files           6        5        -1     
  Lines         516      530       +14     
  Branches       71       91       +20     
===========================================
- Hits          424      318      -106     
- Misses         91      209      +118     
- Partials        1        3        +2     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.