Auto merge of #112697 - tgross35:explain-markdown, r=oli-obk

Add simple markdown formatting to `rustc --explain` output

This is a second attempt at #104540, which is #63128 without dependencies.

This PR adds basic markdown formatting to `rustc --explain` output when available. Currently, the output just displays raw markdown: this works of course, but it really doesn't look very elegant. (output is `rustc --explain E0038`)

<img width="583" alt="image" src="ea418117-47af-455b-83c0-6fc59276efee">

After this patch, sample output from the same file:

<img width="693" alt="image" src="12f7bf9b-a3fe-4104-b74b-c3e5227f3de9">

This also obeys the `--color always/auto/never` command option. Behavior:

- If pager is available and supports color, print with formatting to the pager
- If pager is not available or fails print with formatting to stdout - otherwise without formatting
- Follow `--color always/never` if suppied
- If everything fails, just print plain text to stdout

r? `@oli-obk`
cc `@estebank`
(since the two of you were involved in the previous discussion)

compiler/rustc_session/src/config.rs

@@ -1027,6 +1027,7 @@ impl Default for Options {
             json_future_incompat: false,
             pretty: None,
             working_dir: RealFileName::LocalPath(std::env::current_dir().unwrap()),
+            color: ColorConfig::Auto,
         }
     }
 }
@@ -2807,6 +2808,7 @@ pub fn build_session_options(
         json_future_incompat,
         pretty,
         working_dir,
+        color,
     }
 }