Add single-letter variable warning in JS inlining

[bbugh]

Adds a warning to the interop inlining page about using single-letter variables in JavaScript inlining, to help others avoid getting bitten by mint-lang/mint#452

[gdotdesign]

It's not quite correct, since the variables can be multiple letters like aa, xyz, etc... which could still cause the issue (less likely but still :))

I would say instead:

To be safe prefix your variable names with a dollar sign $.

[Sija]

I'd prefer to fix it on the mint side if possible.

[bbugh]

I agree, and I'm not sure when that might happen. I spent quite a lot of time trying to figure this out especially being new to Mint, and I'd love to say others the trouble. 😅

[bbugh]

Adjusted it for correctness