Add structure rules for package instantiation declarations

[JHertz5]

Subtask of #1306. Implement structure rules for package instantiation declarations.

For example, rules to fix this

package
my_pkg
is
new
my_generic_pkg
  generic map (
    g_my_generic => 2
  );

to this

package my_pkg is new my_generic_pkg
  generic map (
    g_my_generic => 2
  );

[JHertz5]

I've raised a PR https://github.com/jeremiah-c-leary/vhdl-style-guide/pull/1328 to resolve this issue.