Pen: "Stage selected: less pen blocks" label on the stage

[CST1229]

Proposed Changes

Adds a label to the Pen category when on the stage:

Reason for Changes

The motion category already does a similar thing ("Stage selected: no motion blocks"), so this is for parity. Some people have gotten confused about this: https://www.reddit.com/r/turbowarp/comments/1adxxff/why_is_there_only_the_erase_block_in_pen_how_do_i/
And TurboWarp/scratch-blocks#9 got merged, so I assume minor tweaks like this can get accepted. (Though this one adds a string.)

Test Coverage

Haven't touched tests.

[LilyMakesThings]

There are a lot of cases where the stage has less blocks, sometimes for ambiguous reasons. I think this is a good temporary fix but consideration should be made in the future to cover all cases where the stage is an exception to what's included in the categories.