Improve codegen typings

[matthieusieben]

This PR is the first of a 2 steps changes aimed at improving typing in the atproto repo:

• Improve codegen typings #2999 Adds a $type property to records and user object types. Improves the typing of isX and validateX helpers.
• Forbid use of implicit any #2955 forbids "implicit any" in trypescript

Breaking changes

This PR changes the interfaces of the generated code in the following ways:

• it removes the [x: string]: unknown index signature in objects & records. Because of this, access to un-speccified fields must be explicit.
• it makes the $type property explicit (though optional, except in open unions). This allows to properly discriminate entity types, especially when used in open unions.
• it makes the $type property mandatory in record types.
• the isMyType(value) utilities no longer type cast the value being checked. They only type the presence of the $type property.
• validating that an unknown value actually matches a lexicon schema can be achieved using the validateMyType(value) utilities.
• the validateMyType utilities now require that the $type, if present, is valid.
• The new "asPredicate" helper function allows to convert a validateMyType utility into a type predicate like so: const isValidMyType = asPredicate(validateMyType)

Note

When reviewing, "codegen" commits can be ignored as they only contain generated code changes.

Fixes #2952