feat(fs): allow disabling the drag and drop scope extension (#3637)

In plugins/fs/src/lib.rs on_event, every path dropped onto any window is
added to the global runtime scope, directories recursively, with no
opt-out.

Adds the `scopeDroppedPaths` plugin config option (default true, the
current behaviour) so apps can turn it off. Changing the default or
scoping it per window is deferred to v3.

.changes/feat-fs-scope-dropped-paths.md

@@ -0,0 +1,6 @@
+---
+fs: minor
+fs-js: minor
+---
+
+Added the `scopeDroppedPaths` plugin config option (`plugins > fs > scopeDroppedPaths` in `tauri.conf.json`). Set it to `false` to stop adding the paths dropped onto any window to the fs scope, which otherwise gives every webview with an fs permission access to them (directories recursively). Defaults to `true`, the current behaviour.