toolchains: Allow users to provide custom paths to toolchains (#37009)

- **toolchains: Add new state to toolchain selector**
- **Use toolchain term for Add Toolchain button**
- **Hoist out a meta function for toolchain listers**

Closes #27332

Release Notes:

- python: Users can now specify a custom path to their virtual
environment from within the picker.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

Cargo.lock

@@ -17000,10 +17000,15 @@ checksum = "bfb942dfe1d8e29a7ee7fcbde5bd2b9a25fb89aa70caea2eba3bee836ff41076"
 name = "toolchain_selector"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
+ "convert_case 0.8.0",
  "editor",
+ "file_finder",
+ "futures 0.3.31",
  "fuzzy",
  "gpui",
  "language",
+ "menu",
  "picker",
  "project",
  "ui",