Improve tool: add support for multiline comments

Use a more accurate span on assoc types WF checks

Before
```
error[E0275]: overflow evaluating the requirement `<FooStruct as Foo>::A == _`
  --> $DIR/issue-21946.rs:8:5
   |
LL |     type A = <FooStruct as Foo>::A;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
after
```
error[E0275]: overflow evaluating the requirement `<FooStruct as Foo>::A == _`
  --> $DIR/issue-21946.rs:8:14
   |
LL |     type A = <FooStruct as Foo>::A;
   |              ^^^^^^^^^^^^^^^^^^^^^
```

…=dtolnay

Fix Windows Command::env("PATH")

Fixes rust-lang#87859

Link to edition guide instead of issues for 2021 lints.

This changes the 2021 lints to not link to github issues, but to the edition guide instead.

Fixes  rust-lang#86996

… r=notriddle

Fix/improve rustdoc-js tool

This tool is run when testing `src/test/rustdoc-js*`.

r? `@notriddle`