should "a" be represented by dj.substring(0, 1) - or (0,0) ? about djot.js

[FrankFischer]

If you parse the Djot string "ab" ...
was what i wanted to say.

[jgm]

(the char at offset 2) - but this
char does not exist!

Well it does: it's a \n (newline) character.

[jgm]

I'm not really sure what is best. In fact, there are three ways we could go:

1. Current system with the offset of the first character and the offset of the last character
2. Your way, with the offset of the first character and the offset of the character after the last character
3. Offset of first character plus length

Since all the code currently implements 1, we'd need strong reasons to change from that.