Update New Relic version

This change updates the limit of the New Relic version to 3.2.+.

[#62143886]
Add FilteringPathname

This class behaves like Pathname but filters the filesystem and
is optionally immutable.

The implementation leans heavily on method_missing in order to
centralise the filtering, conversion, and mutability logic.

[#61738342]
Prototype enforcing immutability

It is possible to override some common idioms to raise errors when
an attempt is made to mutate the file system using an immutable
FilteringPathname.

This commit contains an example of how to do this.