2.21.0

Various fixes & improvements

• feat(snapshot): Check "blocked" status of iframes before accessing contentDocument (#201) by @billyvg
• fix(canvas): Fix missing addWindow call when enableManualSnapshot==true (#203) by @billyvg
• fix: Move downlevel-dts to devDeps (#204) by @billyvg
• feat(snapshot): Use unpatched setTimeout and clearTimeout (#200) by @billyvg