This is a very minor fixup of #4998 by @klardotsh, just to address the outstanding review comments (code formatting, testing across all ports, etc).

Also enables by default on ESP32 and STM32 (as well as Unix and Windows from the original PR).

Thanks to @klardotsh for doing all the hard work implementing this!! Original PR description below:

This implements (most of) the PEP-498 spec for f-strings, with two
exceptions:

• raw f-strings (fr or rf prefixes) raise NotImplementedError
• one special corner case does not function as specified in the PEP
  (more on that in a moment)

This is implemented in the core as a syntax translation, brute-forcing
all f-strings to run through String.format. For example, the statement
x='world'; print(f'hello {x}') gets translated at a syntax level
(injected into the lexer) to x='world'; print('hello {}'.format(x)).
While this may lead to weird column results in tracebacks, it seemed
like the fastest, most efficient, and likely most RAM-friendly option,
despite being implemented under the hood with a completely separate
vstr_t.

Since string concatenation of adjacent literals is implemented in the
lexer,
two side effects emerge:

• All strings with at least one f-string portion are concatenated into a
  single literal which must be run through String.format() wholesale,
  and:
• Concatenation of a raw string with interpolation characters with an
  f-string will cause IndexError/KeyError, which is both different
  from CPython and different from the corner case mentioned in the PEP
  (which gave an example of the following:)

x = 10
y = 'hi'
assert ('a' 'b' f'{x}' '{c}' f'str<{y:^4}>' 'd' 'e') == 'ab10{c}str< hi >de'

The above-linked commit detailed a pretty solid case for leaving string
concatenation in the lexer rather than putting it in the parser, and
undoing that decision would likely be disproportionately costly on
resources for the sake of a probably-low-impact corner case. An
alternative to become complaint with this corner case of the PEP would
be to revert to string concatenation in the parser only when an
f-string is part of concatenation, though I've done no investigation on
the difficulty or costs of doing this.

A decent set of tests is included. I've manually tested this on the
unix port on Linux and on a Feather M4 Express (atmel-samd) and
things seem sane.