[heft] Introduce a plugin to set environment variables.

[iclanton]

Summary

This PR introduces a new Heft plugin to suppress the warning printed by browserslist when it decides it's out-of-date.

How it was tested

Tested by the build.

[elliot-nelson]

Nice 👍

I think once this merges, we can consider it the official solution to #2981

[octogonz]

Is it better than just having Rush set the environment variable globally for the entire monorepo?

[dmichon-msft]

Needs updating to latest Heft

[octogonz]

Suggest to close this PR and instead implement the two easy features noted in the issue:

#2981 (comment)

[octogonz]

Suggest to close this PR and instead implement the two easy features noted in the issue:

#2981 (comment)

@iclanton and I discussed this in depth. We agreed on the following plan:

1. Add a Rush feature to force environment variables, as proposed in [all] Rushstack CI blocked by "browerslist update" warning #2981
2. Rename heft-suppress-browserslist-warning-plugin to something like heft-env-variable-plugin, that will instead work by forcing environment variables such as BROWSERSLIST_IGNORE_OLD_DATA=1 without specifically focusing on that one package. It can optionally read and apply Rush's configuration.
3. Add a website doc explaining why BROWSERSLIST_IGNORE_OLD_DATA=1 is a best practice for large monorepos

The motivation for implementing both solutions is that Microsoft is worried that even if rush and rushx don't print the warning, if an engineer sees the warning when invoking heft build they may still waste time trying to investigate a non-problem.

[Faithfinder]

Do you think setting the environment variable explicitly should take a priority? Similarly to how dotenv works? Seems like a dev could go into quite a weird debugging session otherwise.

process.env[key] ??= value;

[octogonz]

That's a good idea. Maybe we could split this setting into something like forcedEnvironmentVariables versus defaultEnvironmentVariables. I'm working on a design for common/config/rush/environment-variables.json for Rush that will include some other features, then we can adjust Heft's schema to align with that.