editor: Render boundary whitespace (#11954)

![image](https://github.com/zed-industries/zed/assets/1240491/3dd06e45-ae8e-49d5-984d-3d8bdf98d983)

Added support for only rendering whitespace that is on a
boundary, the logic of which is explained below:

- Any tab character
- Whitespace at the start and end of a line
- Whitespace that is directly adjacent to another whitespace


Release Notes:

- Added `boundary` whitespace rendering option
([#4290](https://github.com/zed-industries/zed/issues/4290)).




---------

Co-authored-by: Nicholas Cioli <nicholascioli@users.noreply.github.com>

crates/language/src/language_settings.rs

@@ -391,6 +391,13 @@ pub enum ShowWhitespaceSetting {
     None,
     /// Draw all invisible symbols.
     All,
+    /// Draw whitespace only at boundaries.
+    ///
+    /// For a whitespace to be on a boundary, any of the following conditions need to be met:
+    /// - It is a tab
+    /// - It is adjacent to an edge (start or end)
+    /// - It is adjacent to a whitespace (left or right)
+    Boundary,
 }
 
 /// Controls which formatter should be used when formatting code.