Add a linter to the language server code itself

[daplf]

As far as I can tell, we're not linting the language server code itself. Perhaps it would be interesting to add this (and add it to the PR builds as well). It would probably make the code a lot cleaner, making it easier to contribute. What do you think?

As for what linter to choose from, suggestions would be welcome, but I suppose we could use ktfmt since it's what we're using to format client code.

[fitrh]

How about ktlint ?

[rami3l]

@fitrh We are in some sort of an awkward situation right now, as @fwcd just made the move from ktlint to ktfmt in https://github.com/fwcd/kotlin-language-server/pull/238.

(Unaware of this move, I suggested ktlint for https://github.com/fwcd/kotlin-language-server/issues/29#issuecomment-1013938293.)

[themkat]

Let's consider this closed by #422