Add ability to warn on missing theme values (#3705)

This PR adds the ability to warn in the `theme_importer` when a theme is
missing values.

Providing the `--warn-on-missing` flag to the `theme_importer` will
print a warning for missing theme value when printing the theme.

```sh
cargo run -p theme_importer -- --warn-on-missing
```

Release Notes:

- N/A

crates/theme_importer/Cargo.toml

@@ -8,6 +8,7 @@ publish = false
 
 [dependencies]
 anyhow.workspace = true
+clap = { version = "4.4", features = ["derive"] }
 convert_case = "0.6.0"
 gpui = { package = "gpui2", path = "../gpui2" }
 indexmap = { version = "1.6.2", features = ["serde"] }