[Nadya]

The proper way in the U.K to denote sarcasm is to use (!) at the end of a sentence. This is done by U.K broadcast subtitles.

For example, watch Dr. Who with subtitles on. You'll see (!) used a lot. :)

The Japanese do use it for sarcasm though I'm not sure how common this is, I've never encountered it: https://en.wikipedia.org/wiki/Tilde#Japanese

It's more common to see "へ～～～" as opposed to "へーーー” (read: "heeeee" in a surprised manner with rising intonation on the e's)

The holding and rising tone makes it also used for happiness and excitement. It's "cutesy" (childlike) because of the held sound.

So ありがとう can become ありがとう～～ (the う is held at the end) which in English would be similar to saying "thank youuuuuuuu!!!"

Instead of "youuuuuuuu" it's been borrowed by some English speakers to replace repeating consonants: "Thank you~"