Update CONTRIBUTING.md to 100 line-length limit (and grammar)

[PallHaraldsson]

[skip ci]

[PallHaraldsson]

I thought of this seeing comment length mentioned here (and the guide is silent on that detail):
#49749 (comment)

I looked up what's common, and The Google Java styleguide is at 100 letters for some reason (and the Linux kernel still in the past at 80 max.).

I'm thinking, we want the shortest vertical space, lines of code possible for functions and modules. But I couldn't find any styleguides mentioning some max there. [And for each individual line, maybe it's best the line excluding whitespace indent is much smaller than 80 letters, but no need to mention that.]

[nsajko]

Why would you want this? It would make it more difficult to have multiple Julia source code tabs open simultaneously.

[KristofferC]

There doesn't seem to be a strong reason to suddenly change this.