Gate pnpm support behind a feature flag to avoid compatibility issues

[charlespierce]

Info

• feat: Initial pnpm support #1273 Added support for the core pieces of pnpm: Instaling, pinning, and running.
• Global package installation / interception are still TODO as we need to work out the specifics of how to implement those.
• As noted in that PR, this change will break the behavior for anyone who was using pnpm as a global package in previous versions of Volta.
• To maintain backwards compatibility, we want to make sure that those users aren't impacted by default.
• We tackled a related issue previously with Yarn 3 support by adding a feature flag environment variable, which should work for this case as well.

Changes

• Added a feature flag environment variable VOLTA_FEATURE_PNPM that will turn on the native pnpm support.
• Updated the install, uninstall, run, and platform checkout flows to gate on the feature flag when handling pnpm
  • If the feature flag is off, we default to the package behavior, which is exactly how pnpm is handled in previous builds.
  • If the feature flag is on, then we use the new behavior that properly supports pnpm

Tested

• Updated the tests to set the environment variable for any pnpm tests
• Manually tested that with the feature flag off, things continue to treat pnpm as a global package
• Manually tested that with the feature flag on, we use the pinned version of pnpm with the new behavior.
• Confirmed that previous builds of Volta will still parse package.json if there is a pnpm value set - though it will be ignored since older versions don't know anything about that key.

Notes

• Once pnpm support is completed with the global package controls, we will likely need some sort of migration to handle users that have pnpm installed as a global package and update their default platform with the appropriate pnpm version.
• We will also remove the feature flag gating at that time and bump the minor version as it represents a new feature.

cc @chawyehsu Who did the initial pnpm implementation as well.

[smblee]

When do we think this will be merged/released?

[mikrostew]

LGTM

[dherman]

LGTM2!! 🚢

[chawyehsu]

Great to see it gets merged. I guess for releasing the next release, there is only #1372 (#1372 (comment)) blocker left now?