Allow table.get() to look up rows by column values

[vishalkaay]

Closes #588.

table.get() currently only accepts a primary key. When a table has an integer primary key but a separate column with a unique constraint (the example from the issue is a collections table keyed by id with a unique name), fetching by that column means falling back to rows_where() boilerplate.

This lets you pass one or more column=value keyword arguments instead:

db["collections"].get(name="entries")

The first matching row is returned, and last_pk is set to that row's primary key so the result can be chained into update()/delete() just like a normal get(). Multiple keyword arguments are ANDed together. Passing both a primary key and keyword arguments raises ValueError, and calling get() with neither raises TypeError. Existing primary-key behaviour is unchanged.

Includes tests (single column, multiple columns, compound-pk and rowid tables, not-found, and the two argument-validation errors) and a new docs section under "Retrieving a specific record".

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📚 Documentation preview 📚: https://sqlite-utils--884.org.readthedocs.build/en/884/