fix: preserve expression aliases when parsing SQL with pl.sql_expr

[alexander-beedie]

Coming at you from several kilometres up in the skies, somewhere over (I think?) Bosnia... :)

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Aliases were getting silently eaten when parsing SQL strings to Polars expressions; parsing as SelectItem instead of bare Expr allows any (optional) aliases to be captured and translated along with the underlying expression itself.

Example

Before:

pl.sql_expr( "MIN(a) as min_a" )
# col("a").min()

After:

pl.sql_expr( "MIN(a) as min_a" )
# col("a").min().alias("min_a")

Also:

Recognise power as an alias for pow ("power" is the usual PostgreSQL syntax, but I believe both are allowed).

[ritchie46]

Still topping our mile high record ;)