Rename BrowserLauncher interface and implementations

Summary:
Renames the **"BrowserLauncher"** concept (`unstable_browserLauncher`) to **"DevToolLauncher"** (`unstable_toolLauncher`), since the scope is now widened to open the desktop debugger shell by default.

Also rename and remove `unstable_` prefixes for `launchDebuggerShell` and `prepareDebuggerShell` methods (APIs remain unstable on the root option key).

**Why not `DebuggerLauncher`?**

This naming remains wide enough to cover other host process delegation in future (e.g. opening other secondary tools, or opening a generic URL in the browser). Debugger launching methods remain specifically named to match their intent.

Changelog: [Internal]

Reviewed By: vzaidman

Differential Revision: D94218654

fbshipit-source-id: e59d5465f200666cd0f1a189414f2223f5e46f69