add go/types based template type checking

[crhntr]

I'd like to do a source based type checker for templates. I intend to add this to muxt generate and have it run at the end.

Here is some related work art:

• https://pkg.go.dev/github.com/jba/templatecheck
• https://pkg.go.dev/github.com/google/safehtml/template

This PR is intended to resolve: #3

Design

The signature of the tree based type checker.

func Tree(tree *parse.Tree, data types.Type, pkg *types.Package, fileSet *token.FileSet, trees TreeFinder, functions FunctionFinder) error

Since, safehtml/template, html/template, and text/template all use text/template/parse, the Tree type from parse makes sense as a parameter to the checker.