Add "--viteMode" CLI flag for dev and build command

[acupofspirt]

Changes

What's inside?

This PR adds a new CLI flag --viteMode for dev and build commands.
The flag value will be passed to the resulting Vite config, allowing us to load different .env.[mode] files. According to Vite documentation, the flag value will also be reflected in import.meta.env.MODE.

What about the existing --mode flag?

It works only for the build command and serves a different purpose than the new flag. --mode should have been called --isProd or --isDev because it sets the internal mode variable, which is heavily used in the codebase to check for production/development mode. Also, it has never been documented, so it can't be considered a public API.

Backward compatibility

As a new feature, it will be a minor change.

Testing

Unit tests were added.

Docs

withastro/docs#8016

/cc @withastro/maintainers-docs

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Latest commit: 40a60c9

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[bluwy]

There's a past PR for this that was closed #6735

EDIT: Well I just noticed that you linked #4850, so we already somewhat support this mode. I don't think supporting custom modes will work everywhere in Astro, but if this is to only align with #4850 then I guess it's fine for now.

[acupofspirt]

@bluwy, you are right. I did it just to get this functional parity between dev and build commands.

[ematipico]

There's a past PR for this that was closed #6735

EDIT: Well I just noticed that you linked #4850, so we already somewhat support this mode. I don't think supporting custom modes will work everywhere in Astro, but if this is to only align with #4850 then I guess it's fine for now.

If what you say is correct, maybe we should add some documentation (considering that we don't have it) and explain expectations. Like, that it won't work everywhere.

[github-actions]

This PR is blocked because it contains a minor changeset. A reviewer will merge this at the next release if approved.

[acupofspirt]

@bluwy @ematipico After a more thorough analysis of the code base I decided to use a new flag instead of the existing one. It will be a cleaner and safer solution. It won't affect internal Astro logic (I've checked all possible places). It just passes the flag value to Vite config.
I've updated the PR description and forcepushed new changes. Later I'll open a PR for the docs.

[sarah11918]

Just a quick changeset edit from docs, and in case wanted, giving you permission to "sell" this feature a little more! 😄