Templates now no longer use Make variables for substitution
but instead replace strings with their equivalent:

  template_name: Corresponds to n=template_name
  project_name:  Corresponds to $(PROJECT) or in=project_name

This allows defining templates outside of Makefiles. For
example an external plugin could define their templates
in templates/my_template.erl and then have the following
in the included Makefile:

  tpl_my_template = $(file < $(THIS)/templates/my_template.erl)

By default the created file will be in src/template_name.erl.
This can be overriden with the tplp_* variable:

  tplp_my_template = src/model/my_template.erl

Substitution is applied both to the template contents and
to its path.

In addition, attempting to overwrite an existing file when
creating a template will result in failure.

[ci skip]

This uses the erlang.mk test infrastructure to build the
package and the packages are installed by default to
XDG_DATA_HOME/erlang.mk/lib. Just add this path to
ERL_LIBS and it will be available to all non-release
Erlang shells.

  export ERL_LIBS=$HOME/.local/share/erlang.mk/lib

To make them available to releases they need to be
started in interactive mode. This can be done via
the CODE_LOADING_MODE variable which is used at the
release startup.

  export CODE_LOADING_MODE=interactive