Adds a way to toggle font size without settings adjustments (#24857)

Closes https://github.com/zed-industries/zed/issues/23505

Now `zed::IncreaseBufferFontSize` (and all the same UI- and
Buffer-related settings) action is parameterized with `{ "persist": true
}` (default).
Using `"persist": false` brings back resizing behavior prior to
https://github.com/zed-industries/zed/pull/23265


Release Notes:

- Added a way to toggle font size without settings adjustments

crates/zeta/src/completion_diff_element.rs

@@ -69,7 +69,7 @@ impl CompletionDiffElement {
         let settings = ThemeSettings::get_global(cx).clone();
         let text_style = TextStyle {
             color: cx.theme().colors().editor_foreground,
-            font_size: settings.buffer_font_size().into(),
+            font_size: settings.buffer_font_size(cx).into(),
             font_family: settings.buffer_font.family,
             font_features: settings.buffer_font.features,
             font_fallbacks: settings.buffer_font.fallbacks,