refactor(api): rename to_* methods for type coercion to something else

[jcrist]

Currently ibis has several to_* methods - some compute and return a result (to_pandas, to_polars, to_parquet, ...), while others are for type coercion and continue to build up an expression (to_timestamp, to_interval, ...).

I think we should rename the latter methods (with a deprecation period), keeping the to_* prefix for things that compute and return a result.

The current list is:

IntegerValue.to_timestamp(self, unit)
IntegerValue.to_interval(self, unit)

StringValue.to_timestamp(self, format_str)
StringValue.to_date(self, format_str)

IntervalValue.to_unit(self, target_unit)

A few options:

• Use as_* prefixes instead. This is what we did for the geospatial API (e.g. as_ewkb, as_binary, ...)
• Use a more specific verb like parse_* for the string ones?
• ?