[build2] How do you specify several file extensions possible?

[Klaim - Joël Lamotte]

I think I saw something like that somewhere in the past but I can't
find an example of this in the current documentation? Maybe I saw it
in one of the dependencies you maintain but I didn't find it again.

Basically while trying to port some code to build2 I need to be able
to express that header extensions can be either hpp, h or inl.

I'm not sure if I understood how to do it properly but would something
like that work?

    hxx{*}: extension = hxx|hpp|h|inl

Maybe an example for this is missing in the doc, I think it is super
common to have C++ codebases with at least 2 kinds of headers.

A. Joël Lamotte