Add approximately pattern

[sffc]

Given https://unicode-org.atlassian.net/browse/CLDR-11431, the following is how we could add the approximately pattern to Intl.NumberFormat:

new Intl.NumberFormat({
  signDisplay: "approximately"
});

Expected outputs:

Input	en-US result with signDisplay: "approximately"
5	~5
0	~0
-0	-0
-5	-5

Alternatively, we could throw RangeError on the negative numbers, since the behavior with them is not well-defined in CLDR.

Related: #6

[littledan]

@sffc This seems like a nice feature. Were you saying there are currently questions about the data quality here?

[sffc]

@sffc This seems like a nice feature. Were you saying there are currently questions about the data quality here?

I'm working on the data quality issues upstream in https://unicode-org.atlassian.net/browse/CLDR-11431

[sffc]

See #24

[sffc]

2021-01-14: We agreed to use the approximately sign as the format when a range collapses down to one number, but not to expose the approximately sign as another option on signDisplay. If we want to add an approximately format in the future, it should be its own option on Intl.NumberFormat, not an overload of signDisplay.