Refactor!: misc. improvements in formatting, type hints, dialect class variables

[georgesittas]

• Dialect properties like time_mapping have been converted to uppercase for consistency, i.e. TIME_MAPPING.
• Dialect properties removed from constructors; they're now set using metaprogramming in the _Dialect class.
• Generator normalize_functions arg has changed: now False means "don't normalize", True is equiv. to "upper".
• Removed the IfNull expression in favor of adding IFNULL as a _sql_name in Coalesce.
• Renamed IS_BOOL generator flag to IS_BOOL_ALLOWED for clarity.
• Tried to replace some Expression constructors with the corresponding helpers in some places.
• Did some manual formatting by adding whitespace to reduce visual noise, unwrapped some multi-line expressions.
• Improved type hints in the Parser.
• Cleaned up some comments.
• Made validate_expression return its argument, to improve composability.

[georgesittas]

Left one final question, but otherwise this PR is ready to go so feel free to merge.

Note: after this is merged I can rebase in #1738 and move the BITWISE to _Dialect, to simplify _parse_bitwise.

[georgesittas]

FYI I reverted the type hint change in the following commit because mypy was yelling
7000a6f#diff-63eb8dd82d3561bc414e3e13cf59516bd6584c5766a10072157a8a5aee4ad85fL3542-R3542

Note that I changed the signature of _parse_bracket in this PR since the previous one didn't seem correct. I think the reason mypy yelled after the rebase is because the main body of the method is also typed now and so type checking actually kicks off. Before we used to have @t.overloads but the actual definition didn't specify types, so my guess is that the method wasn't actually type checked.