Create a new `Template` struct to contain parsing and validation data

[joshuadavidthomas]

There should be a Template data structure that contains:

• Parsed nodelist from the parser
• Some sort of tree structure using Tagspecs for the tags within template
• Validation and errors using Tagspecs
• Template variables contained within template

Plus as a place to put all information output by the djls-templates crate to pass to the server. For instance, knowing the template inheritance graph and all the places the template is included or extended. (Later.. later..)

Previously the Ast struct was going to do this, but I think having that purely about what the parser outputs keeps it a bit simpler from a responsibility standpoint.