add experimental rule to catch missing close brace

[mtoy-googly-moogly]

Adding this rule to the grammar ...

closeCurly
  : CCURLY
  | { this.notifyErrorListeners("'{' missing a '}'"); }
  ;

Makes turns this error message ...

    line 2: extraneous input '<EOF>' expecting {AGGREGATE, CALCULATE, DECLARE, DIMENSION, EXTENDQ,
    GROUP_BY, HAVING, INDEX, JOIN_CROSS, JOIN_ONE, JOIN_MANY, LIMIT, MEASURE, NEST, ORDER_BY,
    PROJECT, QUERY, RUN, SAMPLE, SELECT, SOURCE, TOP, WHERE, VIEW, TIMEZONE, ALL, AVG, CASE, CAST,
    COUNT, DAY, EXCLUDE, FALSE, HOUR, IS, MAX, MIN, MINUTE, MONTH, NOT, NOW, NULL, PICK, QUARTER,
    SECOND, SOURCE_KW, SUM, TABLE, TRUE, WEEK, YEAR, HACKY_REGEX, SQ_STRING, DQ_STRING,
    BQ_STRING, DOC_ANNOTATION, ANNOTATION, '(', '[', '{', '}', '-', ';', LITERAL_TIMESTAMP, LITERAL_HOUR,
    LITERAL_DAY, LITERAL_QUARTER, LITERAL_MONTH, LITERAL_WEEK, LITERAL_YEAR, IDENTIFIER,
    NUMERIC_LITERAL, INTEGER_LITERAL, '"""'}

Into '{' missing a '}'

But I worry that would somehow break the parser for legal sentences, so we might have to back this out.