Update _NAMESPACED_RE in resource_type.py and _resource_type_validation.py
to require both namespace and name components to start with a letter, consistent
with _BUILTIN_NAME_RE and the spec requirement that names start with a letter.
Previously, _NAMESPACED_RE used [a-zA-Z0-9] as the first character class,
allowing digit-starting names like '123abc/my-type'. Now uses [a-zA-Z] to
enforce letter-start for both components.
Also adds Behave scenarios covering rejection of digit-starting custom resource
type names and valid letter-starting names.
ISSUES CLOSED: #2983
freemo