Use "sh" as shell identifier, rather than "shell"

lexers/stxSh.go

@@ -1,10 +1,10 @@
-//go:build stxShell || stxAll
+//go:build stxSh || stxAll
 
 package lexers
 
 func init() {
-	syntaxMap["shell"] = &lazySyntax{init: func() []byte {
-		return []byte(`filetype: shell
+	syntaxMap["sh"] = &lazySyntax{init: func() []byte {
+		return []byte(`filetype: sh
 rules:
     - constant.number: "\\b[0-9]+\\b"
     - statement: "\\b(case|do|done|elif|else|esac|exit|fi|for|function|if|in|local|read|return|select|shift|then|time|until|while)\\b"