Add a mode indicator for vim (#2763)

Release Notes:

- vim: add a mode indicator
([#409](https://github.com/zed-industries/community/issues/409))

Now updated screenshots with @iamnbutler 
<img width="1043" alt="Screenshot 2023-07-25 at 11 11 57"
src="https://github.com/zed-industries/zed/assets/94272/8301479a-8b58-42d8-81a1-bc40e1e0a4df">
<img width="1043" alt="Screenshot 2023-07-25 at 11 12 00"
src="https://github.com/zed-industries/zed/assets/94272/89c3b8bd-9cbc-4fd7-ad10-dac5538ed3a3">
<img width="1043" alt="Screenshot 2023-07-25 at 11 12 12"
src="https://github.com/zed-industries/zed/assets/94272/adc87fe3-a720-4779-853b-df9443407046">

Cargo.lock

@@ -8533,6 +8533,7 @@ dependencies = [
  "indoc",
  "itertools",
  "language",
+ "language_selector",
  "log",
  "nvim-rs",
  "parking_lot 0.11.2",
@@ -8542,6 +8543,7 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "settings",
+ "theme",
  "tokio",
  "util",
  "workspace",