Add rule to detect useless blocks

[ajafff]

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Useless blocks

• don't have a label
• are contained in other block-like Nodes
  • excludes block as children of IterationStatement, IfStatement, WithStatement, TryStatement, ArrowFunction
  • maybe exclude blocks in CatchOrDefaultClause (if it's the only statement in there)
• don't contain block scoped declarations

That idea came up in #103