blank space in ABNF

[cabo]

From the document:

Task (T1): This, besides slice-index, is currently one of only two
places in the document that mentions whitespace. Whitespace needs
to be handled throughout the ABNF syntax. Room Consensus at the
2021-06-15 interim was that JSONPath generally is generous with
allowing insignificant whitespace throughout. Minimizing the impact
of the many whitespace insertion points by choosing a rule name such
as "S" was mentioned. Some conventions will probably help with
minimizing the number of places where S needs to be inserted.

(whitespace is now generally called blank space, but that doesn't change the work to be done.)

[cabo]

Closed by #131 and #132