Turn on ROBOT's label_whitespace check

[hoodcm]

Part of #5.

Check	Data checked	Why
label_whitespace	Labels	A label has no leading or trailing space.

ROBOT's standard check, at its default level (ERROR). Nothing on main fails it, so no data changes. All tests pass.

[jclerman]

This looks like it will work as intended, though it is redundant with the intent of the previous rule. Still, no harm that I can see in adding it.

[jclerman]

I confirmed via testing that this rule fires as expected and would prevent tests from passing in CI, so it should prevent bad values from getting merged into the main branch.

Approved.