LESS embeddable(): Use lessc::toBool

With Ib6bc76736 in place, we have access to lessphp's custom function helpers,
which provide some nice encapsulation for interpreter internals like boolean
nodes. This patch makes embeddable() call $less->toBool() rather than construct
its own AST nodes.

Change-Id: I0a015564dff19f5ced764af182558eb351e3d816
Merge "LESS embeddable(): Use lessc::toBool"