Add Swift and Kotlin templates

[mravn-google]

Go modern with

flutter create --ios-language=swift --android-language=kotlin hello
flutter create --plugin --ios-language=swift --android-language=kotlin hello

Default languages are objc and java.

[mravn-google]

Fixes #7312

[jakobr-google]

Generally LGTM, but could we maybe keep all the files that are identical (icons, etc.) in a common directory?

So you'd have:

ios.tmpl/ -- files shared between Obj-C and Swift templates
ios-objc.tmpl/ -- Obj-C-specific files
ios-swift.tmpl/ -- Swift-specific files

And similar for Android. I think that should work with the current code, and we'd avoid having several copies of especially the binary files.

[mit-mit]

I wonder if we could abbreviate to just: --swift and --kotlin. I guess that wouldn't work if someday there way a single language that both iOS and Android supported, but I don't think that is super likely to happen...

[mravn-google]

@jakobr-google Yes, good point.

@mit-mit There are some minor wrinkles with the simpler --swift and --kotlin approach:

1. As you mention, --some-great-language may at some point become a supported language on more than one platform. Perhaps not very likely now, but we may have additional platforms too.
2. We should probably allow --objc and --java flags as well to allow people to be explicit (e.g. when posting help) and to allow scripts and test code to handle all languages uniformly.
3. Even if not, a third language option on one platform would require us to detect and fail on inconsistencies such as flutter create --kotlin --scala hello. This cross-option consistency condition is a bit awkward to document, and likely leads to duplicated information that must be updated consistently when the language landscape changes.

All minor objections of course (on the other hand, people need to write --android-language only once per project).

[mravn-google]

We might also consider abbreviated forms of the long-hand options, e.g. -al kotlin -il swift.

[mit-mit]

We might also consider abbreviated forms of the long-hand options, e.g. -al kotlin -il swift.

Yes, that sounds like a great compromise!

[mravn-google]

Oh, no. We are not allowed to have two-letter abbreviations.

[mravn-google]

flutter create          -i swift -a kotlin hello
flutter create --plugin -i swift -a kotlin hello

[mit-mit]

Love it, thanks so much!