feat(plugins): when clauses that test whether the project contains a file (#1620)

Plugin when clauses can use workspaceContains:<path>, a path relative to the project root (any project of a workspace). Installation refuses empty, absolute and .. paths. The keys are re-checked on plugin changes, project switches, created or removed files and changes in ignored folders; plugins that don't use the key do no IPC. The key regex avoids lookbehind for WebKit.

Co-authored-by: arnaud-ritti <[email protected]>

plugins/README.md

@@ -152,6 +152,13 @@ is loaded. Menus, shortcuts and status bar items refer to the plugin's
 declared commands. A plugin shortcut never takes a keystroke the IDE (or the
 user) already uses; the user can rebind it in the keymap editor.
 
+A `when` clause (on commands and menus) can test whether the open project
+contains a file or folder: `when = "workspaceContains:.devcontainer/devcontainer.json"`.
+The path is exact (no globs) and relative to the project root; in a workspace the
+key is true when any of its projects contains the path. It updates when
+files change. An IDE that doesn't know the key treats it as
+false: the plugin still installs and the key reads as false there.
+
 ### Activation
 
 Code is instantiated lazily: when one of the plugin's commands runs, or when