Add support for composable schemas in `zed validate`

[tstirrat15]

Description

Part of composable schema support. We want to make it so that zed validate can operate on both composable schemas and standard schemas.

This implements it by adding a new behavior and a new flag:

• By default, validate will attempt to validate a schema first according to composable schema rules, then standard schema rules, and if both fail it will show the errors from composable schema.
• There's a new flag: schema-syntax. If you add --schema-syntax composable, it will validate according to only composable schema syntax rules, and if you add --schema-syntax standard, it will validate according to only standard schema rules. The default value for this flag is the empty string.

I also added proper tests for the validate command, which I'd neglected up until now.

Changes

TODO

Testing

TODO