Hoist indexmap to workspace level (#12901)

This PR hoists `indexmap` up to a workspace dependency.

Release Notes:

- N/A

crates/theme_importer/Cargo.toml

@@ -12,7 +12,7 @@ workspace = true
 anyhow.workspace = true
 clap = { workspace = true, features = ["derive"] }
 gpui.workspace = true
-indexmap = { version = "1.6.2", features = ["serde"] }
+indexmap.workspace = true
 log.workspace = true
 palette.workspace = true
 rust-embed.workspace = true