[Pygments] Complete stubs for Formatter

[brianschubert]

• Complete stubs for Formatter itself
• Add stubs for format and format_unencoded methods in subclasses.
• Removed bad overrides of Formatter class attributes in subclasses.
• Make ImageFormatter inherit from Formatter[bytes] (doesn't support writing to text files)
• Change return type of ImageFormatter.get_style_defs to Never (raises NotImplementedError at runtime)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/_io/terminalwriter.py:223: error: Argument "style" to "TerminalFormatter" has incompatible type "str | None"; expected "type[Style] | str"  [arg-type]

[brianschubert]

The pytest hit looks like a true positive: https://github.com/pytest-dev/pytest/blob/f93656d611f3c9cd22f346dfeda52cddb778c589/src/_pytest/_io/terminalwriter.py#L223

There's no explicit handling for passing None to style, and various Formatter subclasses assume that it is not None (example), so I don't think it can be None in general.

The specific Formatter subclass they're using (TerminalFormatter) never actually uses style itself (and is documented as such), so it's not clear why they're even trying to set it.