The American friends release

Mostly fixes in this updated.

Note the breaking change below...

[0.51.0] - 2024-02-15

Added

• TextArea now has read_only mode #4151
• Add some syntax highlighting to TextArea default theme #4149
• Add undo and redo to TextArea #4124
• Added support for command palette command discoverability #4154

Fixed

• Fixed out-of-view Tab not being scrolled into view when Tabs.active is assigned #4150
• Fixed TabbedContent.TabActivate not being posted when TabbedContent.active is assigned #4150

Changed

• Breaking change: Renamed TextArea.tab_behaviour to TextArea.tab_behavior #4124
• TextArea.theme now defaults to "css" instead of None, and is no longer optional #4157