Fix soft_wrap setting not applying to buffers starting with a different language (#25880)

Closes #22999 
# Problem

Currently, the default soft wrap mode of an editor is determined by
reading the language-specific settings of the language _at offset zero_
in the editor's (multi)buffer. While this provides a way to pick a
single soft wrap mode for a multi-language multibuffer, it's a bad
choice for a single-buffer multibuffer that begins with a different
embedded language. For example, Markdown with frontmatter:

```markdown
---
my_front_matter
---

# Hello World
```

Setting this in config:

```json
  "languages": {
    "Markdown": { "soft_wrap": "bounded" }
  },
```

Will not soft wrap the Markdown file as the language at offset zero is
YAML.

# Solution

Instead of using the language at offset zero, use the language of the
first buffer in the multibuffer (the buffer at offset zero). This gives
better behavior for single-buffer editors, and a similar default for
multi-language multibuffers as before.

# Testing

All existing `editor` crate tests pass, but I would appreciate any
guidance for where best to add additional testing.

Release Notes:

- Fixed soft_wrap setting not applying to buffers starting with a
different language

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/editor/src/element.rs

@@ -4685,7 +4685,7 @@ impl EditorElement {
             .read(cx)
             .buffer
             .read(cx)
-            .settings_at(0, cx)
+            .language_settings(cx)
             .show_whitespaces;
 
         for (ix, line_with_invisibles) in layout.position_map.line_layouts.iter().enumerate() {