Minor code cleanup in jsTyping.ts

[ghost]

• Prefer mapDefined instead of filter(map())
• No need for mutable variables if only assigned to once
• Use a set instead of deduplicate
• Directly add to inferredtypings instead of adding to an array, then merging the array in
• Don't map over an array multiple times, just use a for loop

[jramsay]

do we need a null/undefined check for projectRootPath before it is added to the list of possibleSearchDirs?

[ghost]

It doesn't look like that's ever passed in as undefined?

[jramsay]

At one point I believe there were scenarios where this could be undefined in VS (ex: websites, open folder). Not sure if this is still the case though.

[ghost]

Maybe we should add an assert?