T/4: Initial table support

[jodator]

Suggested merge commit message (convention)

Feature: Initial table support. Closes ckeditor/ckeditor5#3162.

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Additional information

In this PR I've created the fundamentals for table editing:

• upcast conversion
• downcast conversion which supports table editing:
  • inserting table
  • inserting a table row with children
  • inserting a table cell
  • changing table attributes (number of heading rows & columns)

The downcast conversion was tested for upcoming commands that could:

• split table cell
• merge two cells

All above works with colspan & rowspan attributes support.

The commands for those operation were not implemented as it would require additional time to develop them.

Also this PR contains the preliminary implementation of some commands:

• insert table (with basic button UI) - support rows & columns values (UI button always insert 2x2 table)
• insert row (with basic button UI) - command supports insert position & number of row (UI button always insert at the beginningt)
• insert column (with basic button UI) - command supports insert position & number of columns (UI button always insert at the beginning)
  but their code might not be ideal due to focus on conversion itself.

As in ckeditor/ckeditor5#3162 I do not implement or care about:

• cursor movement
• selection
• pasting
• UI
• no widget
• no <p> handling inside table cells
• no caption and no tfoot in view output

[zeopix]

👍

[jodator]

Due to @Reinmar being busy releasing CKEditor 5 v10.0.0 (I love those numbers btw) we've decided to close this PR in favor of one big PR which will cover this (#4) and also table widget (#7) and table commands (#9). The current state of work is on branch t/9 and this will be merged after some commands polishing.