Parser fuzzing: remove previous limitations which do not apply anymore (#19474)

The previous Chevrotain-based parser had a few limitations, which need
to be taken account by the expression generator for the fuzzer. With the
new recursive parser (PR #19335), these limitations are not necessary
anymore:

* leading decimal digit, e.g. `.25`
* nested negatives, e.g. `-- 3`
* nested boolean NOT, e.g. `NOT NOT X`