Define digit in formal grammar

[CAD97]

digit is used as a subtraction in bare-identifier, but never defined.

---

An alternate change:

decimal := integer ('.' digit digit_*)? exponent?
exponent := ('e' | 'E') integer
integer := sign? digit digit_*
digit := [0-9]
digit_ := [0-9_]
sign := '+' | '-'

(NOTE: this has now flip-flopped to opposite what it was when @tabatkins made their comment.)

I think I slightly prefer the PR'd version, but it changes the implied grammar tree structure more significantly. This change "just" gives names to the [0-9] and [0-9_] productions.

[tabatkins]

Afaict both the PR and the alternate change are correct and produce no normative change beyond solving the original issue 👍

I actually slightly prefer the alternate change; I know that I initially missed that an exponent could have a sign when I was implementing, and so I like being more explicit with where exactly a sign is allowed in a number. It also avoids defining a digit_, which I slightly prefer.

[CAD97]

Swapped in the preferred change instead of the less desirable minimal one

[zkat]

can we call this something other than integer? decimal-component or something?