Add support to colorize the output of notes and errors

[estebank]

Sometimes coloring messages could be useful to highlight either inlined code or an important part of the output text without changing the text itself.

[bluss]

rustc's errors are colored. Can you add more information about your configuration, where color is missing?

[nagisa]

rustc --color=always

?

[estebank]

This ticket had a very bad summary, sorry about that. What I'm proposing is adding support to (all?) the note and help methods in Diagnostic, like for example note_expected_found to allow parts of NOTE and HELP lines to be colorized.

The idea to have this came to me while thinking about #21025, as something like this could be accomplished:

foo.rs:9:7: 9:8 error: mismatched types:
 expected `Foo<u8, i8>`,
    found `Foo<u8, i64>`
(expected i8,
    found i64)

[estebank]

Related #21334.