Emit accurate hex colors in generated themes (#3547)

This PR fixes an issues where the hex colors in the generated themes
were not correct.

We're using the [`palette`](https://crates.io/crates/palette) crate to
perform the conversions, as this gives us accurate hex codes that match
the VS Code source themes.

Release Notes:

- N/A

crates/theme_importer/src/vscode/converter.rs

@@ -1,5 +1,5 @@
 use anyhow::Result;
-use gpui::{rgba, Hsla, Rgba};
+use gpui::rgba;
 use indexmap::IndexMap;
 use strum::IntoEnumIterator;
 use theme::{
@@ -7,16 +7,13 @@ use theme::{
     UserHighlightStyle, UserSyntaxTheme, UserTheme, UserThemeStylesRefinement,
 };
 
+use crate::color::try_parse_color;
 use crate::util::Traverse;
 use crate::vscode::VsCodeTheme;
 use crate::ThemeMetadata;
 
 use super::ZedSyntaxToken;
 
-pub(crate) fn try_parse_color(color: &str) -> Result<Hsla> {
-    Ok(Rgba::try_from(color)?.into())
-}
-
 pub(crate) fn try_parse_font_weight(font_style: &str) -> Option<UserFontWeight> {
     match font_style {
         style if style.contains("bold") => Some(UserFontWeight::BOLD),