Implement pasteHandler with plain text fallback

[mkevins]

Description:

Moves forward #624

This PR aims to guard against errors in production during a paste event on mobile. As there are potentially many unexplored code paths (not yet covered by tests), the goal here is to call the original pasteHandler in a try/catch block, and fallback to plain text if an error is encountered.

Note that even in the plain text case, the pasteHandler may still call into some of the complex web code.

Dependencies:

Gutenberg PR: WordPress/gutenberg#14044

Testing:

In order to test, we have to establish a state where a known error exists. A good source to trigger the error is here: #617 (review), but we need to rewind (or revert) the fixes introduced in #630.

1. Roll back to pre Fix for node.rel crash #630 (git checkout 1fead08b5 in gutenberg-mobile and git submodule update)
2. cd gutenberg and cherry pick commit
3. Run demo (may need yarn clean:install first to clear cached dependencies)
4. Follow instructions here: Adds support for pasting in iOS. #617 (review) copying text surrounding 'Gravity Forms'
5. Paste into a new paragraph block

Text should paste as plain text (losing the link and styles). It should not error out to the red screen.

[koke]

Updated PR in #650