LibWeb: Support [SecureContext] extended IDL attribute

[ADKaster]

Many WebIDL interfaces define members that must only be accessed from a secure context.

We do not currently support this attribute, and it is ignored.

The IDL generator should be updated to add checks to ensure that these APIs are unavailable in a non-secure context. This likely involves an audit of all [SecureContext] web apis to make sure that we only define_native_accessor (or other define_native_xyz as applicable) for them when the relevant settings object is a secure context.

In order for this to not cause a mess for our in-tree tests, we'll also need an escape hatch in the Internals object to pretend like we're in a secure context for testing purposes.