fix(number-format): honor min/max-fraction-digits of 0

[spokodev]

The number-format expression dropped min-fraction-digits and
max-fraction-digits when they were set to 0.

At parse time the option is read with a truthiness check:

if (options['max-fraction-digits']) { ... }

0 is a valid value (the style spec documents max-fraction-digits as the
maximum number of fractional digits, and 0 is in range), but it is falsy, so
the option was silently ignored and never passed to the formatter.

Concrete case: ["number-format", 3.7, {"max-fraction-digits": 0}] returned
"3.7" instead of "4". Formatting a value as an integer is a common need for
style authors, and they got the wrong label text with no error.

The fix checks !== undefined for the two numeric options. String options are
left unchanged.

Added an expression test covering max-fraction-digits of 0.

[github-actions]

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