Formatter: handle comments, quotes, and expressions inside f-strings

[dhruvmanila]

This issue is to kickoff a discussion around the formatting for f-strings. With PEP 701, f-strings changes the behavior with what kind of syntax is allowed:

1. Comments inside f-strings:

# comment 0
f"""
{ # comment 1
    # comment 2
    foo # comment 3
    # comment 4
}"""  # comment 5

2. F-strings can be nested within f-strings

f"first {f"second {f"third ..."} second"} first"

3. Quotes can be repeated inside nested f-strings (for both strings and f-strings)

f"double quotes {"same quotes in strings" + f"same quotes in f-strings"}"

This means that the formatter needs to be updated to possibly handle f-strings on it's own. Currently, it's handled as part of string formatting.