Pattern generation: improve inferred map patterns by keeping key order and retaining `_` values

[zachallaun]

Two improvements:

1. If an existing pattern is asserting key presence but ignoring it's value with _, the inferred pattern should also use _
2. When updating an existing pattern, keep the same key order

auto_assert %{foo: 1, bar: _} <- %{foo: "NEW", bar: 2, baz: 3}

# currently defaults to
auto_assert %{bar: 2, foo: "NEW"} <- %{foo: "NEW", bar: 2, baz: 3}

# should default to
auto_assert %{foo: "NEW", bar: _} <- %{foo: "NEW", bar: 2, baz: 3}