Implement a .simple default mode, with *minimum* heuristcs, and only
supporting expansion of @expressions, @include, @if, @else, @for, @end.
The existing .html mode, with its complex state transitions for html/js/css
and heuristics, is now used *only* for template files, that have the `.html`
extension.
vlib is the term for all modules included by default with V and
maintained as part of the V source code repository.
Some included modules depend on third party libraries, and these are kept
separate in the thirdparty directory at the root level of the source
repository.
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