Use official LSP method to fetch content from custom KLS URIs

[fwcd]

Since support for the kls URI scheme (#150) was introduced, the language server has been using the the non-standard protocol method kotlin/jarClassContents to provide source file contents for classes inside JAR archives. This is clearly not ideal, since the one of the main goals of LSP is to provide editor-agnostic support for features such as Go-To-Definition. To work around this issue, the language server has been using temporary files if the editor does not support the kls URI scheme.

There is, however, a new in-progress API (microsoft/language-server-protocol#336) that standardizes this method and we should adopt it once it gets implemented.

[fwcd]

Seems to have been officially upstreamed as workspace/textDocumentContent in LSP 3.18:

• Document text document content request and update changelog microsoft/language-server-protocol#1994

Once LSP4J implements this, we could migrate our non-standard request to this.