feat: Right align line numbers
As part of my work on getting `annotate-snipptes` to be used as `rustc`'s renderer, I realized that `rustc` left-aligned line numbers, while `annotate-snippets` right-aligned them. This PR switches `rustc` to right-align the line numbers, matching `annotate-snippets`. In practice, this change isn't very noticeable in day-to-day output, as it only shows up when a diagnostic span contains line numbers with different lengths (9->10, 99->100, 999->1000, etc.).
`rustc`
```
error[E0412]: cannot find type `F` in this scope
--> $DIR/ui-testing-optout.rs:92:10
|
4 | type A = B;
| ----------- similarly named type alias `A` defined here
...
92 | type E = F;
| ^ help: a type alias with a similar name exists: `A`
```
`annotate-snippets`
```
error[E0412]: cannot find type `F` in this scope
--> $DIR/ui-testing-optout.rs:92:10
|
4 | type A = B;
| ----------- similarly named type alias `A` defined here
...
92 | type E = F;
| ^ help: a type alias with a similar name exists: `A`
```
r? ``@compiler-errors``
15ddf9c7b3