Support using SKIP_JS_INSTALL env variable as an alias for SKIP_YARN_INSTALL/SKIP_BUN_INSTALL

[Fs00]

Support for NPM/PNPM was recently merged, but the name of the environment variable used to skip dependencies installation still refers to Yarn or Bun.
This PR introduces a new unified name for the environment variable: SKIP_JS_INSTALL (which aims to be consistent with SKIP_JS_BUILD introduced in #144), to avoid referring to a specific package manager.
SKIP_YARN_INSTALL and SKIP_BUN_INSTALL are obviously still there for backwards compatibility.

[Fs00]

I've just seen that cssbundling-rails checks those environment variables too.

Should we use different names for the environment variable between the two gems (e.g. SKIP_JS_INSTALL for jsbundling / SKIP_CSS_INSTALL for cssbundling) or do we want to find a common generic name that works for both (e.g. SKIP_DEPS_INSTALL)?
Once we've settled on a name, I can open a PR on cssbundling-rails repo too.