Rather than using dodgy regexes which keep breaking.
Also fixes a regression where it looks like we didn't fail CI if the
delta was in the wrong place.
The schema lint tries to make sure we don't add or remove indices in
schema files (rather than as background updates), *unless* the table was
created in the same schema file.
The regex to pull out the `CREATE TABLE` SQL incorrectly didn't
recognise `IF NOT EXISTS`.
There is a test delta file that shows that we accept different types of
`CREATE TABLE` and `CREATE INDEX` statements, as well as an index
creation that doesn't have a matching create table (to show that we do
still catch it). The test delta should be removed before merge.
