refactor!: Remove old cursor and tab support.

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• FixesDeprecate BasicCursor and TabNavigateCursor #8795
• FixesDeprecate tabbing between text input fields #8792

Proposed Changes

This PR removes the BasicCursor and TabNavigateCursor classes. These provided limited support for keyboard navigation, and have been supplanted by the much-improved experience in https://github.com/google/blockly-keyboard-experimentation. The latter cursor was also used by input fields and blocks to provide support for tabbing between input fields on a block. This was limited to FieldInput subclasses, and has again been supplanted by the keyboard-experimentation plugin.

Breaking Changes

• BasicCursor and TabNavigateCursor have been removed
• Field.isTabNavigable() has been removed
• BlockSvg.tab() has been removed

In all cases, use of the keyboard-experimentation plugin is the recommended migration path.

[rachel-fenichel]

I filed #8821 because we do want to keep some tab behaviour. You can merge this and then re-add tab through the main cursor, or you can do them as separate pieces of work--both work for me.