settings ui: Fix panic from reading BufferLineHeight custom variant (#39631)

The panic happened when a user had a settings file with a buffer line
height custom variant, because the drop-down renderer only took into
account the two named variants.

The fix for this will be creating a custom element that allows a user to
manually input a line height greater than one or select either
Comfortable or Standard.

Release Notes:

- N/A

crates/settings/src/settings_content/theme.rs

@@ -262,18 +262,7 @@ impl From<FontFamilyName> for String {
 }
 
 /// The buffer's line height.
-#[derive(
-    Clone,
-    Copy,
-    Debug,
-    Serialize,
-    Deserialize,
-    PartialEq,
-    JsonSchema,
-    MergeFrom,
-    Default,
-    strum::VariantNames,
-)]
+#[derive(Clone, Copy, Debug, Serialize, Deserialize, PartialEq, JsonSchema, MergeFrom, Default)]
 #[serde(rename_all = "snake_case")]
 pub enum BufferLineHeight {
     /// A less dense line height.
@@ -285,10 +274,6 @@ pub enum BufferLineHeight {
     Custom(#[serde(deserialize_with = "deserialize_line_height")] f32),
 }
 
-impl strum::VariantArray for BufferLineHeight {
-    const VARIANTS: &'static [Self] = &[Self::Comfortable, Self::Standard];
-}
-
 fn deserialize_line_height<'de, D>(deserializer: D) -> Result<f32, D::Error>
 where
     D: serde::Deserializer<'de>,