lua LSP client: initial implementation (#11336)

Mainly configuration and RPC infrastructure can be considered "done". Specific requests and their callbacks will be improved later (and also served by plugins). There are also some TODO:s for the client itself, like incremental updates.

Co-authored by at-tjdevries and at-h-michael, with many review/suggestion contributions.

runtime/autoload/lsp.vim

@@ -0,0 +1,45 @@
+function! lsp#add_filetype_config(config) abort
+  call luaeval('vim.lsp.add_filetype_config(_A)', a:config)
+endfunction
+
+function! lsp#set_log_level(level) abort
+  call luaeval('vim.lsp.set_log_level(_A)', a:level)
+endfunction
+
+function! lsp#get_log_path() abort
+  return luaeval('vim.lsp.get_log_path()')
+endfunction
+
+function! lsp#omnifunc(findstart, base) abort
+  return luaeval("vim.lsp.omnifunc(_A[1], _A[2])", [a:findstart, a:base])
+endfunction
+
+function! lsp#text_document_hover() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/hover', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction
+
+function! lsp#text_document_declaration() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/declaration', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction
+
+function! lsp#text_document_definition() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/definition', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction
+
+function! lsp#text_document_signature_help() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/signatureHelp', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction
+
+function! lsp#text_document_type_definition() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/typeDefinition', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction
+
+function! lsp#text_document_implementation() abort
+  lua vim.lsp.buf_request(nil, 'textDocument/implementation', vim.lsp.protocol.make_text_document_position_params())
+  return ''
+endfunction