Shell: Add support for indexing into variables

Now a variable may have an optional slice (only _one_ slice), which can
also use negative indices to index from the end.
This works on both lists and strings.
The contents of the slice have the same semantics as brace expansions.
For example:
```sh
$ x=(1 2 3 4 5 6)
$ echo $x[1..3] # select indices 1, 2, 3
2 3 4
$ echo $x[3,4,1,0] # select indices 3, 4, 1, 0 (in that order)
4 5 2 1
$ x="Well Hello Friends!"
$ echo $x[5..9]
Hello
```

Userland/Shell/Shell.cpp

@@ -1885,6 +1885,7 @@ void Shell::possibly_print_error() const
     case ShellError::EvaluatedSyntaxError:
         warnln("Shell Syntax Error: {}", m_error_description);
         break;
+    case ShellError::InvalidSliceContentsError:
     case ShellError::InvalidGlobError:
     case ShellError::NonExhaustiveMatchRules:
         warnln("Shell: {}", m_error_description);