refactor(lsp): move more code to client.lua

The dispatchers used by the RPC client should be defined in the client,
so they have been moved there. Due to this, it also made sense to move
all code related to client configuration and the creation of the RPC
client there too.

Now vim.lsp.start_client is significantly simplified and now mostly
contains logic for tracking open clients.

- Renamed client.new -> client.start

test/functional/plugin/lsp_spec.lua

@@ -139,30 +139,6 @@ describe('LSP', function()
       end)
     end)
   end)
-
-  describe('lsp._cmd_parts test', function()
-    local function _cmd_parts(input)
-      return exec_lua(
-        [[
-        lsp = require('vim.lsp')
-        return lsp._cmd_parts(...)
-      ]],
-        input
-      )
-    end
-    it('should valid cmd argument', function()
-      eq(true, pcall(_cmd_parts, { 'nvim' }))
-      eq(true, pcall(_cmd_parts, { 'nvim', '--head' }))
-    end)
-
-    it('should invalid cmd argument', function()
-      eq('.../lsp.lua:0: cmd: expected list, got nvim', pcall_err(_cmd_parts, 'nvim'))
-      eq(
-        '.../lsp.lua:0: cmd argument: expected string, got number',
-        pcall_err(_cmd_parts, { 'nvim', 1 })
-      )
-    end)
-  end)
 end)
 
 describe('LSP', function()