Rollup merge of #126994 - Alexendoo:explain-markdown, r=tgross35

Support lists and stylings in more places for `rustc --explain`

Adds support for `*foo*`, stylings not immediately following whitespace e.g. ``(`Foo`)`` and lists starting with whitespace:

```md
* previously supported
```
```md
 * now also supported
 ```

These are fairly common in the existing error docs, some before/after examples:

### E0460

![image](https://github.com/rust-lang/rust/assets/1830331/4d0dc5dd-b71f-48b1-97ae-9f7199e952ed)
![image](https://github.com/rust-lang/rust/assets/1830331/4bbcb1e4-99ba-4d0d-b338-fe19d96a5eb1)

### E0059

![image](https://github.com/rust-lang/rust/assets/1830331/8457f69a-3126-4777-aa4a-953f7b29f59b)
![image](https://github.com/rust-lang/rust/assets/1830331/ac2189f8-512e-4b3b-886d-6c4a619d17f2)

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -70,4 +70,4 @@ fn spawn<F: Future + Send + 'static>(future: F) {
 
 Similarly to closures, `async` blocks are not executed immediately and may
 capture closed-over data by reference. For more information, see
-https://rust-lang.github.io/async-book/03_async_await/01_chapter.html.
+<https://rust-lang.github.io/async-book/03_async_await/01_chapter.html>.

compiler/rustc_error_codes/src/error_codes/E0378.md

@@ -20,7 +20,7 @@ where
 
 The `DispatchFromDyn` trait currently can only be implemented for
 builtin pointer types and structs that are newtype wrappers around them
-— that is, the struct must have only one field (except for`PhantomData`),
+— that is, the struct must have only one field (except for `PhantomData`),
 and that field must itself implement `DispatchFromDyn`.
 
 ```