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/main.rs

@@ -1,3 +1,4 @@
+mod color;
 mod theme_printer;
 mod util;
 mod vscode;