feat(lib): Add render tag

[YuiYukihira]

Hey, here's my PR for #471.

See the description including

• the selected template cannot access any variables from the parent scope
  • I've created a new OpaqueStackFrame struct, I needed a way to make sure that only the variables passed into the render tag could be accessed from inside it.
• the selected template's variables do not leak into the parent scope
• variables can be passed to the template with either key: value syntax or with with <var> [as <var>]
• rendered content can be looped over with for <value> [as <var>] and the template has access to forloop
  • I moved the ForloopObject and Range structs into a parent module, this is because I wanted to use them for the for variant of the render tag.