Support FORCE_COLOR env var (astral-sh#10839)

Fixes astral-sh#5499 

## Summary

Add support for `FORCE_COLOR` env var, as specified at
https://force-color.org/

## Test Plan

I wrote an integration test for this, and then realized that can't work,
since we use a dev-dependency on `colored` with the `no-color` feature
to avoid ANSI color codes in test snapshots.

So this is just tested manually.

`cargo run --features test-rules -- check --no-cache --isolated -
--select RUF901 --diff < /dev/null` shows a colored diff.
`cargo run --features test-rules -- check --no-cache --isolated -
--select RUF901 --diff < /dev/null | less` does not have color, since we
pipe it to `less`.
`FORCE_COLOR=1 cargo run --features test-rules -- check --no-cache
--isolated - --select RUF901 --diff < /dev/null | less` does have color
(after this diff), even though we pipe it to `less`.

crates/ruff/src/lib.rs

@@ -149,6 +149,13 @@ pub fn run(
     #[cfg(windows)]
     assert!(colored::control::set_virtual_terminal(true).is_ok());
 
+    // support FORCE_COLOR env var
+    if let Some(force_color) = std::env::var_os("FORCE_COLOR") {
+        if force_color.len() > 0 {
+            colored::control::set_override(true);
+        }
+    }
+
     set_up_logging(global_options.log_level())?;
 
     if let Some(deprecated_alias_warning) = deprecated_alias_warning {

docs/faq.md

@@ -628,11 +628,12 @@ Even still, given the dynamic nature of Python, it's difficult to have _complete
 making changes to code, even for seemingly trivial fixes. If a "safe" fix breaks your code, please
 [file an Issue](https://github.com/astral-sh/ruff/issues/new).
 
-## How can I disable Ruff's color output?
+## How can I disable/force Ruff's color output?
 
 Ruff's color output is powered by the [`colored`](https://crates.io/crates/colored) crate, which
 attempts to automatically detect whether the output stream supports color. However, you can force
-colors off by setting the `NO_COLOR` environment variable to any value (e.g., `NO_COLOR=1`).
+colors off by setting the `NO_COLOR` environment variable to any value (e.g., `NO_COLOR=1`), or
+force colors on by setting `FORCE_COLOR` to any non-empty value (e.g. `FORCE_COLOR=1`).
 
 [`colored`](https://crates.io/crates/colored) also supports the `CLICOLOR` and `CLICOLOR_FORCE`
 environment variables (see the [spec](https://bixense.com/clicolors/)).