Add optional support for digit separators and cpp prefixes

[zaewc]

Add optional support in from_chars_advanced to:

• skip a configurable digit separator (e.g. ')
• skip standard cpp prefixes (0x/0X, 0b/0B) before parsing (decimal-only; no hex/binary floats)

Resolves #124

no measurable regression on standard inputs (canada.txt / mesh.txt).

[lemire]

Thanks, to be reviewed.

[lemire]

I rebased the PR here : https://github.com/fastfloat/fast_float/tree/pr-369-rebase

I get that this PR slows down the performance by 5% to 10%.

We could hack things so that the it is performance neutral.

But I am not convinced. I don't know who needs this feature. I am concerned about adding a function that nobody would use but that we would need to maintain.