Mark @filter directive as repeatable in the schema

[obi1kenobi]

Trustfall queries are allowed to have more than one filter on the same field. Such repeatable directives were defined to need a repeatable keyword in the SDL, per a recent GraphQL spec, and our @filter directive should have it.

This would improve compatibility and user experience when using GraphQL editors and LSP integrations, since it will eliminate a source of incorrect "invalid query" errors.

Unfortunately, our current parser does not support repeatable directives so that would be a parse error.

When updating to a newer parser, make the @filter directive repeatable.

[obi1kenobi]

Resolved by #458.