Explore SQL-consistent syntax for joins

[vgapeyev]

PartiQL grammar for join expressions currently allows syntax that is not in SQL:

• OUTER JOIN -- OUTER must come with LEFT, RIGHT, or FULL, not by itself;
• LEFT OUTER CROSS JOIN , RIGHT CROSS JOIN, and other variations of joinType CROSS JOIN -- in SQL, CROSS JOIN is a thing on its own, without further qualification.

While PartiQL sometimes extends SQL syntax in order to anchor constructs with PartiQL-specific semantics distinct from SQL (for example, SELECT VALUE), this does not appear to be the case here. Also, since this involves only the syntactic tokens from the domain of joins -- just in new combinations -- this is particularly confusing/misleading.

Some of this syntax appears in the specification, while there is lore suggesting that LEFT CROSS JOIN is there to work around grammar ambiguity in the presence on the ON condition clause in a join.

The purpose of this ticket it to experiment with the grammar to understand whether there is indeed an issue with SQL-consistent syntax and explore whether it can be resolved. Upon resolution, this will entail adjustments to the specification as well, but those should be purely narrative.

See also partiql-spec #41.