feat: dont underscore numbers when formatting module names

[c43721]

Fixes jsr-io/jsr#803 but I'd like to know how or if I could better test this.

I eventually got here by looking at how the identifier for the file was created:

The identifier replacing all non-characters with underscores seems to be the source so adjusting the regex to allow numbers.

Though this regex might need to account for a leading number and replace that since you can't have a module identifier prefixed by a number (must be a character)

[CLAassistant]

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[c43721]

I'm not sure the regex would want to handle something as complex as "allow a-Za-z0-9 but if the string starts with 0-9 then prepend _" and I'm not sure that would be a valid export so I don't think that should be a concern here. Any inputs?

[crowlKats]

@c43721 yes it should handle the case of a starting number

[crowlKats]

@c43721 for test, you can update the module doc name in tests/testdata/multiple/b.ts