fix Stream's parseFloat()

[d-a-v]

cherry-pick updates for Stream::peekNextDigit() from AVR implementation: decimal dot is optionally allowed

fixes #8782

[earlephilhower]

Apropos of nothing, it's somewhat amusing that the decimal separator is hardcoded to a . by Arduino.cc even though in the Italian locale they use commas for the decimal separator. 😆

[d-a-v]

It's without any regret that I copied this patch.
Decimal commas are hurting computing.

(Note: French decimal separator is also a comma)