[cql2-text] NOT operator is not supported

[gadomski]

Initially raised in stac-utils/stac-fastapi#473, it appears as though the NOT operator is under- or un-supported for CQL2-TEXT. E.g.

collection NOT LIKE 'not-a-collection-id'

#22 appears to be a similar issue that asks about general CQL2-TEXT support, but I thought a specific issue for this problem might be helpful. Please feel free to close if you feel as though this is a duplicate of #22 (or another issue). Thanks!