Assists for adding and maybe removing digit separators from number literals

[srawlins]

As part of #56188, we want to add one or more assists for adding, and maybe removing digit separators (_) from number literals. Here are the cases I've thought of:

Decimal integer literal

Given a decimal integer literal, var x = /*caret*/1000000;, offer to add a separator at 3 digits, yielding var x = 1_000_000;.

If we only offer "every 3 digits" for a decimal integer, then we cannot add any separator for a number with 1, 2, or 3 digits. But what about 4? The Internet has many opinions about whether digit separators should be used for a 4-digit integer. Wikipedia has a good section on Digit grouping. According to a guide from the International Bureau of Weights and Measures, a 4-digit number can feature a separator, but "this practice is not usually followed for numbers having only four digits on either side of the decimal marker except when uniformity in a table is desired." 🤷

I would be ok if we started offering the separators at 4 digits, or at 5 digits.

Hexadecimal integer literal

Given a hexadecimal integer literal, var x = /*caret*/0xff33cc;, offer to add a separator at every 2 digits, yielding var x = 0xff_33_cc;.

Wikipedia mentions that separating every 2 digits, equivalent to each byte, is standard: "Similarly, in hexadecimal (base-16), full spaces are usually used to group digits into twos, making each group correspond to a byte."

Decimal double literal

Given a decimal double literal, var x = /*caret*/12345.67890;, offer to add a separator at 3 digits, both to the left of the decimal, and to the right, yielding var x = 12_345.678_90;.

The same Wikipedia entry, and other areas of the Internet, indicate that commas are not standard on the fractional side of the decimal point, but that other separators like a "thin space", are standard.

Scientific notation double literal

Given a scientific notation double literal, var x = /*caret*/123123.456456e789789;, offer to add a separator at 3 digits, to the left of the decimal, the right of the decimal, and the right of the e, yielding var x = 123_123.456_456e789_789;.

I cannot find any examples or advice regarding separators in the exponent; it seems without evidence to the contrary, we should just be consistent and offer the separators.

Existing separators

When a number has existing separators, I think we should still offer to "add digit separators" to "correct" any "incorrect" ones. It could be worded something like "place digit separator every 3 digits." We could also not offer the assist when the separators are in the expected positions.

Removing separators

It seems to make sense to offer to remove separators. It makes a nice pair of opposing assists. I can also see wanting to remove separators in bulk. Although I think assists cannot currently be offered in bulk.