Clarify what codes can be shortened in the spec

[drinckes]

(From #273 )

Review specification to clarify that all the following are permitted:

Should shortening of codes with only eight digits be allowed? E.g. 8FVC9G8F+
Should shortening of codes with six digits be allowed? E.g., 8FVC9G00+
Should shortening of codes with two or four digits be allowed? E.g., 8FVC0000+

I think we should also specify that padding after the plus e.g. +00G, is not permitted.

[fulldecent]

Here is a draft implementable specification for shortening codes. It clearly answers these questions and provides a clear regex to see what is a short code. (Which is also a useful but not optimized Perl implementation of the isShort function).

https://github.com/google/open-location-code/pull/463/files#diff-467a132a820489de16be4d53f7bd920eccd63b2bed5c986c95bd66064fa3d51cR100-R122

And also at

https://github.com/fulldecent/open-location-code/blob/new-spec/Documentation/Plus%20Codes%20Specification.md?plain=1#L106-L112

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And if you want a different spec, that may be useful just as a starting point.