implement built-in function str_to_date

[cmoog]

Closes #518

This PR implements the STR_TO_DATE MySQL function.

In places where the spec is ambiguous, I'm attempting to match the behavior of MySQL version 8 from my manual testing. I need to implement a few more parsers and add more test cases to cover the expected behavior.

go test  -cover github.com/dolthub/go-mysql-server/sql/parse/dateparse

ok  	github.com/dolthub/go-mysql-server/sql/parse/dateparse	0.163s	coverage: 89.3% of statements

cc @zachmu

[cmoog]

TODO

[cmoog]

@zachmu it doesn't seem I can tag reviewers, but this is ready for a first pass while I finish up the final few format specifiers.

[zachmu]

Overall this looks really good, thanks for the contribution!

The only things missing:

1. An engine test (e.g. enginetest/queries.go)
2. I think the time zone semantics are incorrect. Probably need all results in UTC for this to work reliably for everyone

[zachmu]

I missed the timezone setup in the test, I get it now :)

I'm going to go ahead and merge this in, please add an engine test as a followup. You can implement the missing week specifiers, or leave them as TODO for now. Those are pretty obscure for the purpose of date parsing.