feat!: features option to enable performance impacting non-essential features

[tomasklaen]

Replaces option text_width_estimation, which is now one of its available flags:

features=accurate_text_width

Currently available flags:

accurate_text_width
accurate_sorting

Are there better names for these? Maybe just dropping the accurate_ prefixes, since the "this an improved version of X" is kind of implied already.

[christoph-heinrich]

features doesn't seem right for that. It's not enabling additional features, but instead provides the option for alternative better but slower implementations.

improved_variant=text_width or something like that maybe? Although "improved" is also not really correct, after all if it was a flat out improvement, it would be the default...

[hooke007]

hr.

[po5]

refine=text_width,sorting

[dyphire]

improve_experimental=text_width,sorting
Using experimental descriptions implies that these are not a flat out improvement.
Maybe it can be directly named experimental.