ECHO006 - restricted variable names


why:

  sometimes a project wants to forbid certain variable names.
  this rule makes that configurable.


configure in pyproject.toml:

  [tool.echo-python.echo006]
  names = ["err", "tmp"]
  allow_msg = true


`names` lists extra names to restrict.

`msg` is restricted by default: ruff hard-codes that name for
exception bindings, which the author of this library finds
hateful, so echo-python mutes it out of the box. set
`allow_msg = true` to unmute it.
