gh-87790: support thousands separators for formatting fractional part of floats (#125304)

```pycon
>>> f"{123_456.123_456:_._f}"  # Whole and fractional
'123_456.123_456'
>>> f"{123_456.123_456:_f}"    # Integer component only
'123_456.123456'
>>> f"{123_456.123_456:._f}"   # Fractional component only
'123456.123_456'
>>> f"{123_456.123_456:.4_f}"  # with precision
'123456.1_235'
```