Make unicode-width an optional default dependency

[bjorn3]

Closes #132

[bjorn3]

Can we bump the MSRV to 1.68? That is the first version stabilizing the sparse registry. Due to the lack of sparse registry support the MSRV job currently takes several minutes while all other jobs finish in half a minute.

[the8472]

I haven't found anything explicitly saying what semver policy around default features is, but this could be a breaking change for default-features=false dependents... but currently there only is rustc-dep-of-std which hopefully nobody but std is using, so it's probably fine.

[bjorn3]

The worst thing that can happen when disabling this feature unintentionally is that the help text is twice as wide as it should be causing wrapping inside a terminal.