For include, allow scalars without the file extension

[phudlow]

When using include, barewords are currently allowed:

include foo;

However, when including the same value as a scalar, this isn't allowed:

include "foo";
or
my $foo = "foo"; include $foo;

It becomes necessary to concatenate the extension for the file to be included:

include $foo ~ ".tx";

As a quality of life improvement, just being able to use a scalar of the file name without the extension would be great.