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/Cargo.toml

@@ -13,6 +13,7 @@ gpui = { package = "gpui2", path = "../gpui2" }
 indexmap = "1.6.2"
 json_comments = "0.2.2"
 log.workspace = true
+palette = { version = "0.7.3", default-features = false, features = ["std"] }
 rust-embed.workspace = true
 serde.workspace = true
 simplelog = "0.9"