initial LSP mode support

[christianparpart]

The initial version should be very basic and start with the obviously most useful things that would directly affect the user:

• language diagnostics
• syntax highlighting
• semantic symbol highlighting
• hover tooltips

The LSP mode can be triggered standalone by calling endo --lsp to serve over stdio.

The LSP mode must also be used by endo's input prompt to provide syntax highlighting and realtime language diagnostics etc.

More features that are provided by the LSP protocol (and do make sense for a shell language) may come in future iterations, but the above list is the most important one (for me personally)