pull_requests_fts: 275558612

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275558612	Extract facet code out into a new plugin hook, closes #427	Datasette previously only supported one type of faceting: exact column value counting. With this change, faceting logic is extracted out into one or more separate classes which can implement other patterns of faceting - this is discussed in #427, but potential upcoming facet types include facet-by-date, facet-by-JSON-array, facet-by-many-2-many and more. A new plugin hook, register_facet_classes, can be used by plugins to add in additional facet classes. Each class must implement two methods: suggest(), which scans columns in the table to decide if they might be worth suggesting for faceting, and facet_results(), which executes the facet operation and returns results ready to be displayed in the UI.