Support pasting table with lists content

[mpkelly]

Related to #45774

What?

It's based on requirement 2 from #45774. If you paste a table which includes ul or ol elements, these elements will be converted to simple content and formatted with whitespace to try and keep the original list structure. This avoids having to support complex nested content in tables.

Why?

Increase parity between Gutenberg and other editors.

How?

Allow the list schema to be embedded into td and th. Add a transform function for tables that can convert these tables into simple content.

Testing Instructions

1. Check out this branch and create a table in Google Docs that has ordered or unordered in its cells. See the screenshots below.
2. Paste the table into Gutenberg to see the result
3. Try adding content before/after the list.
4. Try nesting lists to any depth.

Testing Instructions for Keyboard

Screenshots or screencast

Source table (Google docs)

Gutenberg editor after pasting the table above

How the td which contained the list looks in the DOM when previewing

[mpkelly]

@danielbachhuber , @ellatrix. I recreated this PR from the draft #46254. Can you give it a review?

I can beef up the test suite but for now I would like to get some feedback and make sure this PR will be accepted.

[danielbachhuber]

Sweet! This seems to work pretty well to me from a product perspective. I'll defer to others on the codes.

For Example 4 (below), do we want to alternate between letters and numbers?

Example 1

Google Docs

Paste Into Gutenberg

Example 2

Google Docs

Gutenberg Paste

Example 3

Google Docs

Paste Into Gutenberg

Example 4

Google Docs

Paste Into Gutenberg

Example 5

Google Docs

Paste Into Gutenberg

[danielbachhuber]

@mpkelly Can you rebase trunk and make sure all of the tests pass?

[mpkelly]

Thanks for the ping, @danielbachhuber. There are some test failures, but I can't see why. The colspan test I added in a previous PR fails even though that's working fine in the dev environment. Another unrelated test fails due to a difference in how whitespace is encoded. I will try and get everything passing again.