refactor: Move to anstream + anstyle for styling
`rustc` uses [`termcolor`](https://crates.io/crates/termcolor) for styling and writing, while `annotate-snippets` uses [`anstyle`](https://crates.io/crates/anstyle) for styling and currently writes directly to a `String`. When rendering directly to a terminal, there isn't/shouldn't be any differences. Still, there are differences in the escape sequences, which leads to slightly different output in JSON and SVG tests. As part of my work to have `rustc` use `annotate-snippets`, and to reduce the test differences between the two, I switched `rustc` to use `anstlye` and [`anstream`](https://crates.io/crates/anstream) for styling and writing.
The first commit migrates to `anstyle` and `anstream` and notably does not change the output. This is because it includes extra formatting to ensure that `anstyle` + `anstream` match the current output exactly. Most of this code is unnecessary, as it adds redundant resets or uses 256-color (8-bit) when it could be using 4-bit color. The subsequent commits remove this extra formatting while maintaining the correct output when rendered.
[Zulip discussion](https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/annotate-snippets.20hurdles)
Method errors have an extra check that force trim paths whenever the
normal string is longer than 10 characters, which can be quite unhelpful
when multiple items have the same name (for example an `Error`).
A user reported this force trimming as being quite unhelpful when they
had a method error where the precise path of the `Error` mattered.
The code uses `tcx.short_string` already to get the normal path, which
tries to be clever around trimming paths if necessary, so there is no
reason for this extra force trimming.
When the expected and found types have the same textual representation, we add clarifying in parentheses. We now visually highlight it in the output.
Detect a corner case where the clarifying information would be the same for both types and skip it, as it doesn't add anything useful.
Passing an explicit boolean value (`on`, `off` etc.) appears to work, but
actually puts the compiler into an unintended state where unstable _options_
are still forbidden, but unstable values of _some_ stable options are allowed.
Leverage `anstyle-svg`, as `cargo` does now, to emit `.svg` files
instead of `.stderr` files for tests that explicitly enable color
output. This will make reviewing changes to the graphical output of
tests much more human friendly.
according to a poll of gay people in my phone, purple is the most popular color to use for highlighting
| color | percentage |
| ---------- | ---------- |
| bold white | 6% |
| blue | 14% |
| cyan | 26% |
| purple | 37% |
| magenta | 17% |
unfortunately, purple is not supported by 16-color terminals, which rustc apparently wants to support for some reason.
until we require support for full 256-color terms (e.g. by doing the same feature detection as we currently do for urls), we can't use it.
instead, i have collapsed the purple votes into magenta on the theory that they're close, and also because magenta is pretty.
