indent guides: Use primary buffer language to determine tab size (#12506)

When indent guides were still WIP, I thought it might be a good idea to
detect the tab size for every line individually, so we can handle files
with mixed indentations. However, while optimizing the performance of
indent guides I found that getting the language at a given anchor was
pretty expensive, therefore I only resolved the language for the first
visible row. However, this could lead to some weird flickering, where
the indent guides would use different tab sizes depending on the first
visible row (see #12492). This can be fixed by just using the primary
buffer language size.

So as of right now indent guides cannot handle files with mixed
indentations. Im not sure if anyone actually does/expects this, but one
use case I could imagine is something like this:
User x has a svelte file, where the tab size is set to `4`. However the
svelte code uses typescript inside a script tag, which User x wants to
use a tab size of `2`. The approach used here would not work for this,
but then again I think our formatter does not even support something
like this. Im probably overcomplicating things, so let's stick with the
simple solution for now.

Release Notes:

- Fixed an issue where indent guides would use an incorrect tab size
([#12492](https://github.com/zed-industries/zed/issues/12492)).

crates/language/src/buffer.rs

@@ -3153,10 +3153,7 @@ impl BufferSnapshot {
         range: Range<Anchor>,
         cx: &AppContext,
     ) -> Vec<IndentGuide> {
-        fn tab_size_for_row(this: &BufferSnapshot, row: BufferRow, cx: &AppContext) -> u32 {
-            let language = this.language_at(Point::new(row, 0));
-            language_settings(language, None, cx).tab_size.get() as u32
-        }
+        let tab_size = language_settings(self.language(), None, cx).tab_size.get() as u32;
 
         let start_row = range.start.to_point(self).row;
         let end_row = range.end.to_point(self).row;
@@ -3167,9 +3164,6 @@ impl BufferSnapshot {
         let mut result_vec = Vec::new();
         let mut indent_stack = SmallVec::<[IndentGuide; 8]>::new();
 
-        // TODO: This should be calculated for every row but it is pretty expensive
-        let tab_size = tab_size_for_row(self, start_row, cx);
-
         while let Some((first_row, mut line_indent)) = row_indents.next() {
             let current_depth = indent_stack.len() as u32;