customized TextIndex tracking

[OmarTawfik]

This type is already tracking three values (utf8, utf16, char), and two more with #683 (line, column). is it worth it to only allow a subset through Cargo features, thus preserving memory usage?

Once we have #808, we should probably measure its impact and decide if we should support this:

• For Rust users, they can enable/disable the values they care about.
• For TS users, can we choose a reasonable default. For example, is utf8 ever used/needed in NodeJS?

If perf shows that there is no measurable difference between 1 and 5 values, then it is a good reason to skip this.

[Xanewok]

We need numbers, but I don't think this is worth pursuing for Rust as the cost will probably be negligible; maybe more so for TS users.