Add tasks/commands for generation of the l10n from the .arb files

[TiffApps]

Is your feature request related to a problem? Please describe.
We always have to manually generate/regenerate the .arb files with a command in the terminal whenever we edit them, or we clone/pull a project.

Describe the solution you'd like
Like the task in pubspec.yaml that auto runs the command flutter pub get when we save it, I'd like to auto run the command flutter gen-l10n every time I save my .arb files. And probably also an icon at the top right of .arb files that does the same, again like the pubspec.yaml. Also, running this command whenever we clone/pull a project (like it does with flutter pub get) would be nice.

Describe alternatives you've considered
Using the command flutter gen-l10n in the terminal everytime we edit .arb files, or clone/pull a project.

Additional context
None.

[DanTup]

I've added a new Task and Command (Flutter: Generate Localizations) for this. It'll also show up in the context menu for .arb files (in both explorer and the editor) and in the menu bar when editing a .arb file.

I haven't yet added support to run it on-save, I've split that into #3993.

[hyungtaecf]

I added the Run On Save plugin on VS Code, and then added the following settings to VS Code's settings.json:

"emeraldwalk.runonsave": {
    "commands": [
      {
        "match": "\\.arb$",
        "cmd": "flutter gen-l10n"
      }
    ]
  },

It works perfectly. 👍

[DanTup]

In the next release, #3993 adds a new setting that works similar to hot-reload-on-save that will trigger the built-in Flutter: Generate Localizations command: