Spirit does not support CREATE INDEX statement with a functional index

[kolbe]

	abstractStmt, err = New("CREATE INDEX idx ON t1 ((`a` IS NULL))")

github.com/block/spirit/pkg/statement.convertCreateIndexToAlterTable({0x10362f2e0?, 0x140001ae510})
	/Users/kolbek/Devel/git/cashapp/spirit/pkg/statement/statement.go:246 +0xb8
github.com/block/spirit/pkg/statement.New({0x10344d9d5, 0x26})
	/Users/kolbek/Devel/git/cashapp/spirit/pkg/statement/statement.go:67 +0xc4
github.com/block/spirit/pkg/statement.TestExtractFromStatement(0x1400012c8c0)
	/Users/kolbek/Devel/git/cashapp/spirit/pkg/statement/statement_test.go:70 +0x738
testing.tRunner(0x1400012c8c0, 0x1036200a0)
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1934 +0xc8
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1997 +0x364

[kolbe]

The panic is fixed by #445 but CREATE INDEX with a functional index is simply rejected now to avoid the issue. Someone could improve the support for CREATE INDEX so that it's rewritten correctly.