Support IF NOT EXISTS for table creation

[rafguns]

Currently, I have a bunch of code that looks like this:

subjects = db["subjects"] if db["subjects"].exists() else db["subjects"].create({
    ...
})

It would be neat if sqlite-utils could simplify that by supporting CREATE TABLE IF NOT EXISTS, so that I'd be able to write, e.g.

subjects = db["subjects"].create({...}, if_not_exists=True)

[simonw]

This is a good idea. We already have that parameter for the table.create_index() method: https://sqlite-utils.datasette.io/en/stable/reference.html#sqlite_utils.db.Table.create_index

[simonw]

Just noticed there's no explicit test coverage for the db["table"].create(...) method.

[simonw]

Updated documentation is here: https://sqlite-utils.datasette.io/en/latest/python-api.html#explicitly-creating-a-table