feat(bigquery): Add parsing support for STRPOS(...)

[VaggelisD]

This PR adds parsing support for exp.StrPosition in BigQuery, enabling it's transpilation:

bigquery> SELECT STRPOS('foo@example.com', '@');
4

snowflake> SELECT POSITION('@', 'foo@example.com');
4

duckdb> SELECT STRPOS('foo@example.com', '@');
┌────────────────────────────────┐
│ strpos('foo@example.com', '@') │
│             int64              │
├────────────────────────────────┤
│                              4 │
└────────────────────────────────┘

Docs

BQ | DuckDB | Snowflake