added initial languageserver connection

[Tomatower]

Let me begin implementing #16 (althought it is not yet merged upstream...)
This is my first - and last - typescript/vscode extension I will ever write, so any help is greatly appreciated!

• Add connection to a running OpenSCAD instance
  • Receive Diagnostics
  • Trigger a preview in the linked instance (registered as command openscad.lsp.preview)
• Query the installed openscad version if it does/does not support the LSP magic (check if it starts with command line flag --lsp-listen, which is the switch for the languageserver)
• If the detected version supports lsp, change all existing commands that currently start a new openscad instance to start relay to the linked instance. (this will require more support upstream, but thats what I am here for)
• Add the option to Download the latest release / current snapshot build.

closes #16

[Antyos]

Do you want me to merge this now or wait until you are done?

If you want, I think I can also add you as a collaborator on the repo so you can commit to a branch directly instead of having to work off of a fork.

[Tomatower]

I hope to get some more input from your side how we best align the user interface - the LSP does change a lot of the interaction with the running program.

It would be cleaner to work from a branch in this repo I think, so this would be nice!

[Antyos]

I just added you as a collaborator, so you should have an invite in your email.

I also changed the PR to commit into a new branch I made for your work on the language server.