Implement == and != operators for AExpressions.

Refs #5789
Implement support for local variables in AExpressions.

Refs #5789
Implement the 'apply template' directive.

Refs #5789
Allow arrays as operands in constant expressions.

Refs #5789
Make Value operators const where possible.

Refs #5789
Implement additional operators: &&, ||, in and !in.

Refs #5789
Fix the !in operator.

Refs #5789
Make braces around constant expressions optional.

Refs #5789
Implement regex() and match() functions.

Refs #5789
Update documentation.

Refs #5789
Merge branch 'feature/apply-5789' into next

Fixes #5789