no-loop-var-prefix: refactor to use matchtask()

[nishipy]

A part of #2105.

This PR refactors the no_loop_var_prefix rule and resolves the following problem. Please refer to here.

• currently uses matchplay() with custom match methods: handle_play(), handle_tasks(), handle_task().
• unlike the standard match*() methodds, the custom block detection in this rule is incomplete: it only descends into block, not rescue or always.
• the handle_*() and matchplay() methods should be condensed into a single matchtask().

[nishipy]

I will wait until the schema update gets merged