fix: better paste handling

[YousefED]

• Fixes pasting from an external spreadsheet into an existing table
• closes Pasting text pastes it inside a new block #1198 (and closes Copy pasting an Emoji insert a page break #1321, probably same)
• Fixes pasting text in Custom Blocks. For example, pasting text in an empty (no-text) alert block would previously replace the entire block (fixed by setting the custom block as isolating). Described in this issue: Typed or Copied Content Replaces Custom Alert Block Instead of Staying Within It #771 (but doesn't fully close it as the issue is also about keyboard shortcuts)

TODO:

• fix build
• add unit tests for (a) pasting tables (b) 1198 (c) pasting inside a custom block

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[YousefED]

some small comments. Did you also check the tests without the code changes? (i.e.: validated whether the tests are indeed going from "wrong snapshot" to "correct snapshot" when enabling the fixes)?