Add a text-to-number coercion

[jahav]

This was not fun.

I can just image a that conversation when they were writing the specification:

Technical writer: So I got a task to document type coercion for the Open XML standard. How does it work?
Dev: That's a deep hole. We have number, fractions, currencies, percents, time spans and dates. We use our own parser that accepts a lot of inputs that are just not generally considered fine, like 1000 hours in a date and so on.
Technical writer: Can we utilize some other standards?
Dev: Not really, this goes way back to first versions in 1980s and 90s, there wasn't even unicode back then. Each country had it's own separate build with a baked-in culture, so it couldn't change parsing methods on the fly.
Technical writer: What if I open an excel from US that has a date in a format 'MM/dd/yyyy' and open it different country with a different date format?
Dev: Yeah, the date won't be coerced to number in the new country.
Technical writer: Are date formats described in some other official specification? ISO8601... anything?
Dev: Don't think so. Just our internal docs.
Technical writers: What about numbers? They have pretty well established grammar.
Dev: Yeah, there are some not-so-accepted features, like a allowed whitespacespace between a sign and a number, like "- 1"
Technical writer: So the coercion is a culture specific and doesn't conform to any kind of public standard, thanks to compatibility going back decades.
Dev: That pretty much sums it up.
Technical writer: Screw it. This will do.

An implementation is permitted to provide an implicit conversion from string-constant to number. However, the
rules by which such conversions take place are implementation-defined. [Example: An implementation might
choose to accept "123"+10 by converting the string "123" to the number 123. Such conversions might be locale-
specific in that a string-constant such as "10,56" might be converted to 10.56 in some locales, but not in others,
depending on the radix point character. end example]

Related to #1891.