Make LspTextCommand inherit from sublime_aio.ViewCommand

[rwols]

This PR makes LspTextCommand inherit from sublime_aio.ViewCommand. The benefit is that this ViewCommand has special handling for async def run() methods. So, you can write directly such a method, without having to use the somewhat cumbersome pattern of defining an async def _run() method and invoking that one, via run_coroutine, in the regular run() method. So:

# OLD
FooBarCommand(LspTextcommand):
    def run(self, edit: sublime.Edit, foo: str, bar: str) -> None:
        run_coroutine(self._run(foo, bar))

    async def _run(self, foo: str, bar: str) -> None:
        ...

# NEW
FooBarCommand(LspTextCommand):
    async def run(self, foo: str, bar: str) -> None:
        ...

However, I see TypeErrors being thrown for old LspTextCommand-derived classes that don't have an async run() method. This PR is blocked on that: packagecontrol/sublime_aio#34

[jwortmann]

I was thinking about whether we could make a command wrapper that would allow to report back a result when (asynchronically) running a command. Then we could use that in a way like

result = await run_command(view, 'lsp_some_command', {})

instead of the regular ST API call (which always returns None)

view.run_command('lsp_some_command', {})

This could be useful for example when applying TextEdits or a WorkspaceEdit (e.g. LspApplyWorkspaceEditCommand), where it would be nice to know whether an operation failed, or to get back the number of applied edits and edited files.