[FR]: Allow using BuildSettingInfo flags for env vars

[titanous]

What is the current behavior?

Currently the only straightforward way that I'm aware of to propagate variables (API URLs, etc) into builds is by using --define with the variable interpolation in the env attribute. --define is not recommended according to the Bazel docs:

--define is only supported for backwards compatibility. Prefer Starlark build settings whenever possible.

Describe the feature

webpack_bundle and webpack_devserver should have a env_settings or equivalent attribute that takes a settings label to env var map, like the old esbuild rule from rules_nodejs has:

load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
string_flag(
    name = "api_endpoint",
    build_setting_default = "https://example.com/v1",
)
esbuild(
    name = "bundle",
    define_settings = {
        ":api_endpoint": "API_ENDPOINT",
    },
)

I think this would require adding the same attribute to js_run_devserver, as webpack_devserver is a macro and so wouldn't have access to the flags during the loading phase.

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