feat: add OptionSetNumberSeparator to format numbers with separators (#222)

[littfed]

Fixes #222

Description

This PR adds support for formatting numbers with custom digit separators (such as _, ,, or spaces).

Features Added

• OptionSetNumberSeparator(separator string): sets the digit group separator.
• Aliases: OptionSetSeparator and OptionSeparator for convenience.
• Formats iteration counts (current/max and unbounded counts) and iteration rates (it/s, it/min, it/hr).
• If no separator is configured (default), numbers remain formatted without separators, preserving backwards compatibility.

Example

bar := progressbar.NewOptions(1048576,
    progressbar.OptionSetWidth(10),
    progressbar.OptionShowCount(),
    progressbar.OptionSetPredictTime(false),
    progressbar.OptionSetNumberSeparator("_"),
)
bar.Add(1024)
// Output: 0% |          | (1_024/1_048_576)

Testing

• Added TestFormatNumber table-driven unit tests covering edge cases (single digits, exact multiples of 3, large numbers, negative values, various separators).
• Added TestOptionSetNumberSeparator covering the option and aliases.
• Added ExampleOptionSetNumberSeparator runnable example test.
• All existing tests pass cleanly.