`yegappan/lsp` support

[Song-Tianxiang]

add support to vim9script lsp client yegappan/lsp

[Shougo]

@uga-rosa What do you think?

[uga-rosa]

I thought it was time to take a look, but it looks like development has stopped. Hmmm, I don't know what to do.

[Shougo]

Oh....

[Shougo]

@Song-Tianxiang If you really need the feature, you can contribute it.