Don't silently ignore tokens from themes without a name

[danbulant]

If we pass in a JSON theme and want to highlight tokens inline (the thing{:.tok} syntax), this requires a name even if just one theme is passed in. There's no warning or error about name not being present, and everything else works just fine.

Relevant code: https://github.com/rehype-pretty/rehype-pretty-code/blob/master/packages/core/src/index.ts#L309-L320