Use official LSP method to fetch content from custom KLS URIs

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 (https://github.com/microsoft/language-server-protocol/issues/336) that standardizes this method and we should adopt it once it gets implemented.

Start by reviewing the current kotlin/jarClassContents method and the temporary-file workaround described in the issue, then read the linked LSP API discussion. Done means the language server uses the standardized method for custom KLS URIs instead of the non-standard method and workaround.

Written by the indexing model from the issue text.