Investigate improving configuration flow by sending `workspace/configuration` at startup

[fwcd]

Extracted from https://github.com/fwcd/kotlin-language-server/pull/367#discussion_r969869742

There was some discussion upstream (in the LSP repo) on encouraging servers to move to a pull-model where the server fetches configurations through workspace/configuration at startup instead of relying on the configuration to always be passed at launch. Ideally we should probably do something similar in the medium term.

Some interesting threads:

• https://github.com/microsoft/vscode-languageserver-node/issues/524
• https://github.com/Microsoft/language-server-protocol/issues/567
• https://github.com/haskell/haskell-language-server/issues/2762

[NepNep21]

So, i am actually working on that, however with the way the configuration class currently works it would be quite clunky to implement, what do you suggest as a rough idea? This is how the request and its handling would generally work with the current system:

private fun newConfig(item: String) = ConfigurationItem().also { it.section = item }

override fun initialized(params: InitializedParams?) {
    client.configuration(ConfigurationParams(listOf(
        newConfig("foo"),
        newConfig("bar")
    ))).get().let { // Blocking to ensure we are loaded before continuing
        // Note the boilerplate and inability to use a loop as well as the order requirement
        config.foo = it[0] as X
        config.bar = it[1] as Y
    }
}

Reflection could potentially be used to make it better but that would still be fairly hacky, i believe it would be better to rewrite the config system entirely.