Add methods to get position from column and line in TextEdit

[YuriSizov]

This closes godotengine/godot-proposals#1644 for master. I'm going to make another PR for 3.x, but it's going to be quite different for obvious reasons.

The added methods can be useful to draw something around specific characters. We'll have a proper API for that at some point, but this should be generally useful still. And this would allow to introduce similar methods to 3.x and preserve feature parity.

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I'm really interested to see if this is a good implementation, as I don't feel comfortable working with TextServer yet and am not confident about the approach 😅

[YuriSizov]

I've applied suggested changes and removed some no-longer-necessary code. Retested with folding and wrapping and it seems to work as expected.

Curious about the "If the first and last character is visible, it doesn't necessarily mean all character in between are" case, but don't know what to do about it. We can remove the check for the visible range, of course. It was only proposed as an optimization step to avoid going for characters that are definitely not visible (as for the purposes of these new methods, we return (-1, -1) for those).

So please let me know what I need to do here.

[Paulb23]

Couple nitpicks otherwise looks good.

If the col optimisation is too much effort to work in, will be okay to leave it out.

[YuriSizov]

I've moved things around as requested, and added a note to the docs to explain the difference between values returned by the two methods. Since I want to keep get_pos_at_line_column, I didn't feel like renaming as suggested would work with the existing get_line_column_at_pos.

PS. See the second to last force-push for the relevant changes, the last one is just a rebase.

[akien-mga]

Thanks!