Support for code completion in the scope provider API

[loradd]

Code completion requests could be answered using the scoping rules defined through a language-specific scope provider. In order to enable such feature, ScopeDescription must support searching for all symbols names - possibly filtered through a text string. I suggest adding the following method to ScopeDescription:

fun allVisibleNames(filter: String = ""): Sequence<String> = sequence<String>{
    // include names from all local nodes in this scope
    namesToLocalSymbolNodes.keys.filter { it.contains(filter) }.forEach { yield(it) }
    // include names from all external symbols in this scope 
    namesToExternalSymbolIdentifiers.keys.filter { it.contains(filter) }.forEach { yield(it) }
    // include all visible names from the parent scope (if any)
    parent?.allVisibleNames(filter)?.also { yieldAll(it) }
}

Furthermore, the ScopeProvider should provide support to retrieve scopes from string property names and node types, e.g. scopeFor(nodeType: String, propertyName: String): ScopeDescription?. In this way, we would need to retrieve Node instances before asking for the scope of a given reference - which would be cumbersome in the language server and require some sort of node repository access before. This feature would enable computing scopes from information contained in SymbolDescriptions.

[ftomassetti]

This may be useful indeed