[Proposal] Refactor formatting logic to use external command(s)

[rami3l]

After reading several issues about code formatting support (#29, #303, #304), I think it might be feasible to just make a configurable subprocess command and launch it when code formatting is triggered, in the following steps (to be completed):

1. Fix the default formatting style (of the plugin and of the codebase): Fix code formatting with ktfmt (fixes #304) #330.
2. Add option to customize formatting: TODO.

This proposal is made with the following considerations in mind:

• There are various ways of formatting a Kotlin codebase. Apart from the ktlint vs. ktfmt debate, each has different installation options (for just Gradle for example, there are existing Gradle plugins like ktlint-gradle and ktfmt-gradle, and there's also spotless).

• This has been done in similar projects. For example, rust-analyzer simply launches rustfmt to handle this.

• However, given the fact that there is no rustfmt equivalent in the Kotlin world, and that some might want to use this plugin without an explicit per-project formatter config, it's acceptable to keep the embed ktfmt as the default formatter.

What do you think of this idea? :)

[fwcd]

I think that's a great idea. Can external formatters handle things like ranged formatting? KLS uses a virtual file system, but perhaps there are CLI formatters that work e.g. on Kotlin code passed to stdin.

[rami3l]

@fwcd
Ah it's complicated... I thought that ktfmt would have something like --lines but it's an open issue at facebook/ktfmt#218.

So google/vim-codefmt#183 used a workaround which might be interesting to look at...