Be more accurate about calculating display_col from a BytePos

No longer track "zero-width" chars in `SourceMap`, read directly from the line when calculating the `display_col` of a `BytePos`. Move `char_width` to `rustc_span` and use it from the emitter.

This change allows the following to properly align in terminals (depending on the font, the replaced control codepoints are rendered as 1 or 2 width, on my terminal they are rendered as 1, on VSCode text they are rendered as 2):

```
error: this file contains an unclosed delimiter
  --> $DIR/issue-68629.rs:5:17
   |
LL | ␜␟ts␀![{i
   |       -- unclosed delimiter
   |       |
   |       unclosed delimiter
LL | ␀␀  fn rݻoa>rݻm
   |                ^
```

compiler/rustc_span/src/source_map/tests.rs

@@ -232,7 +232,6 @@ fn t10() {
         source_len,
         lines,
         multibyte_chars,
-        non_narrow_chars,
         normalized_pos,
         stable_id,
         ..
@@ -246,7 +245,6 @@ fn t10() {
         CrateNum::ZERO,
         FreezeLock::new(lines.read().clone()),
         multibyte_chars,
-        non_narrow_chars,
         normalized_pos,
         0,
     );