typescript/leap: Add mentor notes

[SleeplessByte]

Adds mentor notes based on the JavaScript notes, but greatly extended for typescript:

• tslint mentions,
• links to typescriptlang.org's handbook,
• TypeScript specifics such as typing functions, type inference and boxed types.

[sshine]

Very nice!

I think the last example has "by10" when "by100" was meant. Update: Also, are there cases of "is" with accent signs over the I, or am I getting short-sighted?

[SleeplessByte]

@sshine That was definitely oversight 😅

Update: Also, are there cases of "is" with accent signs over the I, or am I getting short-sighted?

Yes, I think there are two instances of ís instead of is. I commonly use it (but also see it in the wild) when someone like myself wants to set something up to have extra emphasis.

In this case:

tslint will not warn when mixing && and ||, which ís a feature of eslint

It's also present in other notes but I'll gladly replace it with is or is, if that's what you prefer.