More bugs eliminated! + some tiny enhancements

- Added CTRL+Z to undo the last destroy!! Now you can bring things back if you accidentally delete the whole page.

- Fixed some sizing bugs so animations are smoooooother when destroying things.

- Added a fade effect to the current target to make it clearer what will be deleted.

- More ways to get things destroyed:
-> Start destroying by right-clicking anywhere (in addition to CTRL+SHIFT+L or clicking the icon)
-> Get out of it by right-clicking or hitting Q (in addition to ESC or clicking the icon)
-> Destroy elements by pressing D, X, Enter, Backspace or Delete (in addition to just clicking!)

- Update to Manifest V3: If everything worked, you won't even notice! Just keep up with browser changes.

DESTROYED SOME BUGS!!!

- Elements with `position: fixed` are now targeted correctly.

- Element borders could sometimes cause incorrect offsets with the previous position calculator. The calculator now uses shiny new browser features to get it 100% correct 100% of the time (hopefully!)

- iFrames (often used to hold ads) used to catch clicks before Dom Destroyer, which could sometimes result in clicking on the ad when you wanted to destroy it. Dom Destroyer now slips a shim in front while in targeting mode, so that you can click stuff to destroy without fear!

- Container elements with the same size and shape as their children are now auto-destroyed! Fewer clicks to make things go _all the way_ away.