Add @{escapedVariable} in selector functionality - selector interpolation

[SomMeri]

(~"h1 @{escapedVariable}") syntax is deprecated in less.js. They will use @{escapedVariable} placed directly in the selector.

New syntax needs to be implemented and the old one should remain working and produce warning.

Less.js references:

• Variable property less/less.js#36
• Support strings as property names less/less.js#698

[SomMeri]

Sub-tasks:

• warning on deprecated (~"syntax"),
• warning on deprecated @variable as pseudo class parameter,
• strict parsing - mixins references must not allow interpolation.

[SomMeri]

Implemented including warnings and strict parsing.