V2: Rename JSON serialization option emitDefaultValues

[timostamm]

The documentation for the suggested JSON options has been updated recently. The documentation originally only applied to proto3, where the name emitDefaultValues fits. With Editions, this name has become ambiguous: The option does not actually emit field default values (which would change data with a serialization roundtrip), it only emits empty maps and lists, and zero values for proto3 singular scalar fields (fields with implicit presence).

This PR renames the option to alwaysEmitImplicit, and updates the documentation accordingly:

/**
 * By default, fields with implicit presence are not serialized if they are
 * unset. For example, an empty list field or a proto3 int32 field with 0 is
 * not serialized. With this option enabled, such fields are included in the 
 * output.
 */
alwaysEmitImplicit: boolean;