text/v2: utility APIs for text fields

[hajimehoshi]

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

What feature would you like to be added?

If an Ebitengine user wants to implement a text inputting field, these features are needed:

• An API to get an text index from a position
• An API to get a position from a text index

Probably these are considered in text/v2 (#2454).

Other things that are already filed are:

• An API to render a text with specifying alignments (text: a utility function to render texts with a left-upper position #2143)

Why is this needed?

These are useful to implement text fields. I actually tried to do it, and found these are necesasry, but the implementation was tricky.

[hajimehoshi]

See #2827