feat: add handle_continue/2 to GenLSP behaviour

[Moosieus]

Why:

• Follow this conversation.
• Long story short, writing the initialization response needs to block everything thereafter to be compliant with LSP spec.
• Mostly impacts VSCode AFAIK.

Changes:

• Mirrors GenServer.handle_continue/2 as close as possible.
• If applicable, waits for the principal callback's response to be written (not responded to!) to the buffer before invoking handle_continue/2.

Test branch w/ Expert available at Moosieus/expert/tree/moo/work-done-progress-handle-continue.

[mhanberg]

@Moosieus I think the purpose this solves is actually also solved by #78. This feature might be useful in the future but I'm going to close for now since it's not necessary.