Improve highlighting errors

[Nacho Cordón]

At the moment we are providing errors with a very simple syntax listener that admits for improvements (wrong position for the error sometimes):

export class ErrorListener implements ANTLRErrorListener<CommonToken> {
  diagnostics: Diagnostic[];

  constructor() {
    this.diagnostics = [];
  }

  public syntaxError<T extends Token>(
    recognizer: Recognizer<T, any>,
    offendingSymbol: T | undefined,
    line: number,
    charPositionInLine: number,
    msg: string,
    _: RecognitionException | undefined,
  ): void {
    const lineIndex = (offendingSymbol?.line ?? 1) - 1;
    const start = offendingSymbol?.startIndex ?? 0;
    const end = (offendingSymbol?.stopIndex ?? 0) + 1;

    const diagnostic: Diagnostic = {
      severity: DiagnosticSeverity.Warning,
      range: {
        start: Position.create(lineIndex, start),
        end: Position.create(lineIndex, end),
      },
      message: msg,
    };
    this.diagnostics.push(diagnostic);
  }
}

The messages are unclear and they offer too many keywords most of the time:

[Nacho Cordón]

Moved to Trello!