Rename `incdec.pl.m4` to `incdec.m4.pl`

Like we did for the `incdec.m4` Bash include, rename the Perl M4 file to
give it a `.pl` extension so we get Perl syntax highlighting. Similarly
define an include macro so quoting doesn't mess up highlighting.

1 files changed, 4 insertions, 0 deletions

diff --git a/incdec.m4.pl b/incdec.m4.pl
new file mode 100644
index 0000000..46e9a7e
--- /dev/null
+++ b/incdec.m4.pl
@@ -0,0 +1,4 @@
+INCLUDE_INCDEC_SUBROUTINE
+
+my $output = incdec($line, $increment_by, $point_position, $backward);
+print $output;