Shell: Parse a pipe sequence inside $(...)

Author: https://github.com/alimpfard
Commit: 42304d7bf1
Pull-request: https://github.com/SerenityOS/serenity/pull/2542
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Shell/Parser.h

@@ -125,7 +125,8 @@ expression :: evaluate expression?
             | comment expession?
             | '(' list_expression ')' expression?
 
-evaluate :: '$' expression {eval / dynamic resolve}
+evaluate :: '$' '(' pipe_sequence ')'
+          | '$' expression          {eval / dynamic resolve}
 
 string_composite :: string string_composite?
                   | variable string_composite?