Add text based cursor movement helpers

[lhecker]

COOKED_READ_DATA is a little special and requires cursor navigation
based on the raw (buffered) text contents instead of what's in the
text buffer. This requires the introduction of new helper functions
to implement such cursor navigation. They're made part of TextBuffer
as these helpers will get support graphemes in the future.
It also helps keeping it close to TextBuffer as the cursor
navigation should optimally behave identical between the two.

Part of #8000.

[DHowett]

Now, what is position. A column or a string index?

[lhecker]

Ah, good point. I forgot to add function documentation. 🙂
(It's a string index, since it's for text-based navigation.)

[DHowett]

I am weirded out that they're in TextBuffer but I think i understand why

[lhecker]

I am weirded out that they're in TextBuffer but I think i understand why

Once the implementation for these gets beyond simple surrogate pairs, it'll probably be more obvious. 😅 After all, both ROW and COOKED_READ_DATA will then use the same underlying text parsing algorithm. It's all still in flux, but I think I'll end up writing a TextAnalyzer class/namespace in the future which replaces CodepointWidthDetector. It'll then be used by the two classes instead of these new functions. But since such a class/namespace doesn't exist yet, I figured the next best thing is to at least keep it close to TextBuffer.

[carlos-zamora]

Approving. Please be sure to add the function comment though.