Fixes bug with '(' showing for allShortestPaths, ANY, etc

[ncordon]

What

Solves a bug where we get allShortestPaths '(' instead of allShortestPaths as completion.

Why

This was introduced when the errors code started to use c3 for giving better error messages. The reason is the c3 engine keeps a static copy of the ATN cached for all the instances, that depends only on the name of the parser, but not the ignoredTokens:

export class CodeCompletionCore {
    private static followSetsByATN = new Map<string, FollowSetsPerState>();

The ATN transitions get computed having into account the ignored tokens:

    if (!this.candidates.tokens.has(symbol)) {
        // Following is empty if there is more than one entry in the set.
        this.candidates.tokens.set(symbol, set.following);

There's a race condition between the errors code (which doesn't configure ignored tokens to get brackets in the error messages :Expected one of (, [, etc) and the completions engine (which just suggests keywords).