feat: paste into correct workspace, allow cross-workspace pasting

[maribethb]

Fixes #615

• Removes the copyWorkspace property from the clipboard actions. This is now (once again) tracked in core for the default copy/paste implementation, and having it here was incorrect for the cross-tab-copy-paste implementation.
• By default, the "paste" option in the context menu will be greyed out if you try to paste in a workspace that does not match the one that was copied from. This is because if you use the default implementation of copy/paste from core, you will always paste into the workspace that was copied from, and it would be confusing if you selected the "paste" option in workspace B but a block appeared in workspace A.
• If a developer has enabled an alternate implementation of copy/paste, such as that provided by @blockly/plugin-cross-tab-copy-paste, then they can pass an option upon initializing the plugin that makes the option not appear greyed out in these cases.
• Updates the readme with this information

Tested with vanilla blockly and with the cross-tab-copy-paste plugin.

Note to reviewer: I rearranged the code in clipboard.ts because it was in an order that was driving me nuts. If you view this PR commit-wise it will be easier to review.