Add test for 'environment variable'

Part of #4363

Author: coke

xt/word-variants.rakutest

@@ -25,6 +25,7 @@ my %variants = %(
     # see for example doc/Language/js-nutshell.rakudoc
     Boolean    => rx/ << boolean <!before \s* <[ \= \< \> \{ \} ]> > /,
     "call site" => rx/ <<'call' '-'? 'site'>>/,
+    "environment variable" => rx:i/ << environmental [\s+|\-] variable/,
     filehandle => rx:i/ << file [\s+|\-] handle /,
     filesystem => rx:i/ << file [\s+|\-] system /,
     lookahead  => rx:i/ << look \- ahead /,