feat: add a cmd to request window size

[aymanbagabas]

Fixes: #987 (comment)

[atticus-sullivan]

Because the PR got merged only a few hours ago and this issue is 100% certain linked to this PR, I'm writing no new issue but comment on this PR.

Sorry for waiting until after the merge before testing this. In my test, when I send a tea.WindowSize() in the Update() function of a model, it causes the program to hang up. My analysis shows that the reason behind this most probably (well tbh, I'm pretty certain on this) is because
you call p.checkResize() in the eventLoop() which in turn calls p.Send() which in the end sends on the p.msgs channel. The problem is (by default) this channel is not buffered, thus the send blocks until another goroutine reads from it (but the one which would read from it is the one sending on that channel).

I'm not too familiar with the codebase but searching for other occurrences of p.Send(), it seems like this function is called in a new goroutine usually. In this case I'm not sure if the p.checkResize() should call go p.Send() (would also affect the init stuff which I'm not familiar with) or if we should call go p.checkResize() in the eventLoop().

I hope I didn't formulate this too complicated otherwise feel free to ask me what I actually mean. Any thoughts on this? Also, didn't someone else had this issue as well?

[aymanbagabas]

@atticus-sullivan Thank you for noticing this one! I've opened #1059 to fix this one and add an example to request window size.

[atticus-sullivan]

Alright, thanks for the fast reply + fix. Also having an example for this is quite nice 👍