Make minimum width for line numbers in gutter configurable (#31959)

Closes #7334

# Changes
This PR makes the minimum width allocated for line numbers in the side
gutter configurable in units of character width via the
`"line_number_base_width"` attribute in `gutter` settings. Set the
previously hard coded value of `4` as default.

Together with other settings (`"folds"`, `"breakpoints"`,...) this gives
the user control over the gutter width.

If the number of lines exceedes the base width, the number of digits in
the largest line number is chosen instead. This is consistent with
previous behaviour.

Screenshot for reference:
<img width="1104" alt="Screenshot 2025-06-03 at 12 15 29"
src="https://github.com/user-attachments/assets/77c869ad-164b-4b74-8e39-8be43d740ad4"
/>


P.S.: This is my first time contributing to zed (yay!🎉). Let me know if
i'm missing something.

Release Notes:

- Make minimum line number width in gutter configurable

assets/settings/default.json

@@ -445,7 +445,9 @@
     // Whether to show breakpoints in the gutter.
     "breakpoints": true,
     // Whether to show fold buttons in the gutter.
-    "folds": true
+    "folds": true,
+    // Minimum number of characters to reserve space for in the gutter.
+    "min_line_number_digits": 4
   },
   "indent_guides": {
     // Whether to show indent guides in the editor.