Allows customizing the executable and arguments used by
Tools → Launch Copilot CLI (External Terminal) via a new VS Unified
Settings category (Settings → Copilot CLI IDE Bridge → External
Terminal). Defaults preserve existing behavior (`cmd.exe /k copilot`).

Supports any shell (`wt.exe`, `pwsh.exe`, `powershell.exe`, custom
path) and a `{WorkspaceFolder}` placeholder for terminals like Windows
Terminal that don't inherit the parent process working directory.
The resolved command/arguments/cwd are written to the Copilot CLI
output pane on launch for diagnostics.

Also:
- Rename existing Terminal settings category to "Embedded Terminal"
- Refactor TerminalSettings to share GetString/GetInt32 helpers
- Refactor TerminalSettingsProvider to derive moniker from key (ToMoniker)
  removing duplicate moniker constants

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>