Allow sending text after `shrug`

¯\_(ツ)_/¯ IDK, I like to write something after, sometimes

Allow sending:

¯\_(ツ)_/¯ some text

By entering:

/shrug some text

It's supported in Element. If you did it in nheko, you just used to lose the
extra text.

Not putting text is still supported, a ternary operator makes sure a
space is only inserted if the text isn't empty, to avoid having a trailing
space.