feat(duckdb): support transpilation of REGR_VALX and REGR_VALY from snowflake to duckdb

[fivetran-kwoodbeck]

DuckDB doesn't support these functions but a simple CASE statement makes them work.

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:feature/transpile-regression, sqlglot version: feature/transpile-regression)
• baseline (main, sqlglot version: 28.3.1.dev18)

⚠️ Limited to dialects: snowflake, bigquery, duckdb

By Dialect

dialect	main	sqlglot:feature/transpile-regression	difference	links
bigquery -> bigquery	2576/2621 passed (98.3%)	2576/2621 passed (98.3%)	No change	full result / delta
bigquery -> duckdb	1058/2620 passed (40.4%)	1071/2620 passed (40.9%)	⬆ improved by 0.5%	full result / delta
duckdb -> duckdb	3558/4003 passed (88.9%)	3549/4003 passed (88.7%)	⬇ regressed by 0.2%	full result / delta
snowflake -> duckdb	311/1085 passed (28.7%)	311/1085 passed (28.7%)	No change	full result / delta
snowflake -> snowflake	975/1085 passed (89.9%)	975/1085 passed (89.9%)	No change	full result / delta

Overall

main: 11414 total, 8478 passed (pass rate: 74.3%), sqlglot version: 28.3.1.dev18

sqlglot:feature/transpile-regression: 11414 total, 8482 passed (pass rate: 74.3%), sqlglot version: feature/transpile-regression

Difference: No change