Support Snowflake-specific EXTRACT syntax

[seve-martinez]

Per the Snowflake docs, EXTRACT functions can be written one of two ways:

SELECT
  EXTRACT(hour from foo)
FROM bar

SELECT 
  EXTRACT('hour', foo) -- or unquoted hour
FROM bar

This will result in a parse error:

ParseError(ParserError("Expected: date/time field, found: 'hour' at Line: 14, Column: 13"))

The parse_extract_expr() method on the Parser explicitly expects a FROM keyword:
https://github.com/sqlparser-rs/sqlparser-rs/blob/1e209d87415a5adfedccac8cee3e2860122e4acb/src/parser/mod.rs#L1682-L1692