1. add dedicated `meta.directive` scopes (like in VueJS syntax)
2. use angular expression syntax in directive values instead of plain JavaScript

This commit moves angular expressions for a more logical order of sections,
starting with...

1. HTML injections
2. angular directives
3. angular declarations (let statement)
4. angular control flow statements
5. angular interpolation

followed by...
6. angular expressions
   with all following contexts being part of expressions.

This commit adds `.expression` sub-scope name to distinguish embedded
interpolation expressions from control flow statements.