Update and rename perl6.yaml to raku.yaml (#1927)

The language has been renamed but still aims to support the old file extensions for a time.

runtime/syntax/raku.yaml

@@ -1,7 +1,7 @@
-filetype: perl6
+filetype: raku
 
 detect: 
-    filename: "(\\.p6$|\\.pl6$|\\.pm6$)"
+    filename: "(\\.p6$|\\.pl6$|\\.pm6$|\\.raku$|\\.rakumod$|\\.rakudoc$)"
 
 rules:
     - type: "\\b(accept|alarm|atan2|bin(d|mode)|c(aller|h(dir|mod|op|own|root)|lose(dir)?|onnect|os|rypt)|d(bm(close|open)|efined|elete|ie|o|ump)|e(ach|of|val|x(ec|ists|it|p))|f(cntl|ileno|lock|ork)|get(c|login|peername|pgrp|ppid|priority|pwnam|(host|net|proto|serv)byname|pwuid|grgid|(host|net)byaddr|protobynumber|servbyport)|([gs]et|end)(pw|gr|host|net|proto|serv)ent|getsock(name|opt)|gmtime|goto|grep|hex|index|int|ioctl|join|keys|kill|last|length|link|listen|local(time)?|log|lstat|m|mkdir|msg(ctl|get|snd|rcv)|next|oct|open(dir)?|ord|pack|pipe|pop|printf?|push|q|qq|qx|rand|re(ad(dir|link)?|cv|do|name|quire|set|turn|verse|winddir)|rindex|rmdir|s|scalar|seek|seekdir|se(lect|mctl|mget|mop|nd|tpgrp|tpriority|tsockopt)|shift|shm(ctl|get|read|write)|shutdown|sin|sleep|socket(pair)?|sort|spli(ce|t)|sprintf|sqrt|srand|stat|study|substr|symlink|sys(call|read|tem|write)|tell(dir)?|time|tr|y|truncate|umask|un(def|link|pack|shift)|utime|values|vec|wait(pid)?|wantarray|warn|write)\\b"