feat(lsp): initial support for textDocument/rename

flow keybinds: changes f2 from toggle_input_mode to rename_symbol and
moves toggle_input_mode command to ctrl+shift+f2 (since ctrl+f2 is
already bound to insert_command_name)

the replacement text is hard coded for now. i've checked that replace
works with zls and pylsp which send WorkspaceEdit response messages in
different shapes - zls sends shape `{"changes": {}}` while pylsp sends
`{"documentChanges": []}`.

currently the 'rename_symbol_item' commands are sent one at a time.
however they should be buffered and be performed between one
buf_for_update, update_buf pair.  this will be addressed in a follow up.

src/keybind/builtin/flow.json

@@ -128,7 +128,8 @@
             ["shift+enter", "smart_insert_line_before"],
             ["shift+backspace", "delete_backward"],
             ["shift+tab", "unindent"],
-            ["f2", "toggle_input_mode"],
+            ["f2", "rename_symbol"],
+            ["ctrl+shift+f2", "toggle_input_mode"],
             ["ctrl+f2", "insert_command_name"],
             ["f3", "goto_next_match"],
             ["f15", "goto_prev_match"],