Add multi-line var support

[kapodes]

Currently #var can only support one line keys. So same long prefix yaml paths have to be copied multiple times.

#var foo.bar.baz: 123
#var foo.bar.bas: 456

it would be nice to add native yaml multi-line support for yaml.

[kapodes]

Adding new @var syntax that supports multi-line notation. So example from issue becomes

@var:
  foo:
    bar:
      baz: 123
      bas: 456

this notation adds 2 properties to context that can be then referenced by placeholders as usual.

keyBaz: ${component-name@foo.bar.baz}
keyBas: ${component-name@foo.bar.bas}