Use shell to launch MCP and ACP servers (#42382)

`npx`, and any `npm install`-ed programs, exist as batch
scripts/PowerShell scripts on the PATH. We have to use a shell to launch
these programs.

Fixes https://github.com/zed-industries/zed/issues/41435
Closes https://github.com/zed-industries/zed/pull/42651


Release Notes:

- windows: Custom MCP and ACP servers installed through `npm` now launch
correctly.

---------

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

Cargo.lock

@@ -3595,6 +3595,7 @@ dependencies = [
  "settings",
  "smol",
  "tempfile",
+ "terminal",
  "url",
  "util",
 ]