Memory leak on disposing targets or load project

[SimonShiki]

Expected Behavior

The target's data should be clear when it get disposed.

Actual Behavior

Memory leak happens on both disposing targets or load project.

Steps to Reproduce

1. Open DevTools, Shot a memory heap snapshot.
2. Open a large project, like https://scratch.mit.edu/projects/613688710
3. Shot again.
4. Create a new project, shot again.

Screenshots

(Init -> Load an large project -> Create a new project)

The Cause

1. _StackFrameFreeList never get clear

_StackFrame holds target, and _StackFrameFreeList will recycle the disposed stack frames. but it never get clear.
A verified fix here: Clipteam/clipcc#237

2. Previous target's skins won't get disposed

This problem has been addressed before (an issue in old scratch-gui repo). A workaround can be considered:

        if (this.renderer && '_allSkins' in this.renderer) {
            this.renderer._allSkins.forEach(skin => {
                this.renderer!.destroySkin(skin._id);
            });
        }