Move validators into core spec, rethink suppressions

[mtdowling]

Move validation into core

This commit moves model validation and suppressions into the core.
EmitEachSelector and EmitNoneSelector are now part of smithy-model and
were removed from smithy-linters. The `@suppress` trait was added to
suppress validaation events on specific shapes. The `validators`
metadata property was updated so that it now only take `id`,
`namespace`, and `reason`, where `namespace` can be set to `*` to
suppress a validation event for all namespaces or validation events that
aren't specific to a single shape.

The ability to add custom suppressions to the ModeAssembler has been
removed.

Move validation into core spec

This commit moves validation concepts into the core Smithy spec while
leaving more specific linters in a linters guide.