This PR greatly increases the utility of @plugin in the following ways:

1. Custom functions can be called from the root of a ruleset. This is so plugins can return any type of node, not just a Value node. Less still checks during evaluation if the node is allowed there. So, for example, if no custom function is defined, then Less will assume it's an unknown CSS function, which is not allowed in the root.
2. Like mixins, custom functions can receive detached rulesets, to do more advanced transformations of rules via JavaScript, or: to simply pass "structured data" to a function. Or, to pass a Less list and another node type to do some kind of processing on each member of the list as it relates to that Node. There really isn't a limitation here.
3. Functions now allow semi-colon separators. This addresses Allow Semicolon as Function Argument Separator #1505 and a number of the use cases in Allow parametrized mixins as detached rulesets to form 'lambdas' #2270.

I've added some additional tests demonstrating custom functions.