Allow SQLITE_MAX_VARS to be customized via Database(sqlite_max_vars=...)

[AmadNaseem]

Refs #147.

SQLITE_MAX_VARS is hard-coded to 999, which caps how many rows insert_all batches into a single INSERT. Many SQLite builds are compiled with a much higher SQLITE_MAX_VARIABLE_NUMBER (e.g. 250,000 on Debian/Ubuntu, 500,000 on recent macOS), so on those systems the 999 cap forces many more, smaller batches than necessary.

As suggested in the issue, this adds an optional sqlite_max_vars argument to the Database constructor:

db = Database("data.db", sqlite_max_vars=250_000)

• Default behaviour is unchanged: when the argument is not given, the limit falls back to the module-level SQLITE_MAX_VARS (999).
• A Database.sqlite_max_vars property exposes the effective value.
• Table.insert_all uses it in both places that previously referenced the module global: the column-count guard and the batch-size calculation.

I deliberately kept this to the constructor argument only, and did not add automatic detection of the compiled limit — that would change default batching for everyone and is a larger, separate change.

Docs updated in docs/python-api.rst; tests added in tests/test_create.py (default stays 999, a raised value produces fewer INSERT batches as measured via the tracer hook, and the column-count error message reflects the custom value).

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📚 Documentation preview 📚: https://sqlite-utils--784.org.readthedocs.build/en/784/