feat(snowflake)!: Annotate type for VAR_POP, VAR_SAMP, DuckDB consistency fix for VAR_SAMP

[fivetran-kwoodbeck]

Mainly just tests for Snowflake, but in testing I noticed an inconsistency in naming on duckdb. It uses the alias of VAR_SAMP called VARIANCE. I made it consistent so it's VAR_SAMP and VAR_POP, instead of VARIANCE and VAR_POP. sqlglot/dialects/duckdb.py references a VARIANCE_POP which does not seem to exist.