Tracks Grid block enhancements and bug fixes targeting 7.0.

Enhancements

• Enable grid block to be responsive when columns are set
• Disable left and top drag handles when in Auto mode
• Nested Grid blocks - show only one grid visualizer at a time

Bug fixes

• Prevent Grid Column Width Exceeding Container - PR: Prevent Grid Column Width Exceeding Container (Grid interactivity) #69059 and Ensure grid column never exceeds parent's width #74795
• Possibly Not able to select paragraph (P block) text inside CSS Grid Block cell if we're able to work out what the issue is there.
• Number of columns in Grid Block seems incorrect in themes with default blockGap greater than 1.2rem (see #73662)
• Hide grid visualiser if the grid block is hidden
• Allow grid to use style variation blockGap values for columns calculation