Shell: Initial support for 'option' completions

Take one small step towards #2357.
Handle completing barewords starting with '-' by piping the requests to
the Shell::complete_option(program_name, option) :^)

Also implements completion for a single builtin (setopt) until we figure out how
to handle #2357.

Shell/Builtin.cpp

@@ -776,3 +776,16 @@ bool Shell::run_builtin(int argc, const char** argv, int& retval)
 
     return false;
 }
+
+bool Shell::has_builtin(const StringView& name) const
+{
+#define __ENUMERATE_SHELL_BUILTIN(builtin) \
+    if (name == #builtin) {                \
+        return true;                       \
+    }
+
+    ENUMERATE_SHELL_BUILTINS();
+
+#undef __ENUMERATE_SHELL_BUILTIN
+    return false;
+}