feat(app-vite, app-webpack): configure env files programatically

[jclaveau]

This gives all the flexibility required to fully customize the process of loading env files like

• customizing mode names
• adding a staging vite-mode like forseen here Env Variables and Modes #15303 (comment)
• following the Quasar app-mode convention with custom vite-modes as the context is provided
• give a dedicated place to this kind of logic in the conf

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
  • => I have no Cordova App to test it
• It's been tested on an Electron app
  • => I have no Electorn App to test it
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.
  • => The types have been updated, I guess it will update the docs

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information:

I have not been able to find any unit test for the config so I assumed it is not tested