Element-typed component inputs (array of string)

[taras]

Problem

Component input types stop at string | number | boolean | array | object | any — an array input cannot declare its element type. From review of
ReleaseSpecWarning (files is an array of path strings): "can we say array
of strings here?" An unknown type string fails every validation, so this is
not expressible today.

Proposal

Element-typed arrays in InputDefinition + validate.ts (syntax open:
array<string> or items:), with spec coverage in executable-mdx-spec and
tests. ReleaseSpecWarning.files becomes the first user.