Add trusted-set-constant scriptlet

Alias: `trusted-set`

Behaves exactly like set-constant, except that any arbitrary JSON-
compatible value can be set.

By default the value is treated as a string, which can be anything.

If the value starts with `{` and ends with `}`, the value will be
JSON-parsed, and the `value` property of the resulting object will
be used.

As with any scriptlet requiring trust, filters using
`trusted-set-constant` can only come from trusted filter lists,
otherwise they are discarded.

Related discussion:
- uBlockOrigin/uAssets#18185 (comment)