Whether a value is a path that can only lead somewhere on this site.
Parameters
path: string
A candidate path, without its query string
Returns boolean
true when the path is absolute and free of unsafe syntax
Remarks
The shared first check behind every next-style return parameter: it must
be absolute, and it must not carry anything a browser could read as a
different origin or a climb out of the path.
Whether a value is a path that can only lead somewhere on this site.