perf: Optimise DASH manifest generation

[absidue]

This pull request introduces various optimisations to DASH manifest generation. I have checked that the generated XML is still the same as before.

1. It turns out that the JSX transform only checks the first letter in the tag name to decided if it is an element (lower case) or a component/function (upper case). So I decided to change the tag names to be the same as their actual names but with the first letter changed to lower case, that way we can simplify the code in normalizeTag name to just change the first character to upper case.
2. For text nodes we currently create a TEXT_ELEMENT wrapper object and then give that TEXT_ELEMENT object special treatment in renderElementToString to treat it as a string. As our target is just a string and not some DOM like environment, I simplified it to just pass through the string and then to give it special treatment if it is a string. That means the code is simpler and we are creating less temporary objects.
3. I simplified the property handling from using .filter(), .map(), checking the .length, then calling .join() and appending it to the element string to a for-of loop with an if that appends directly to the element string.

The DASH manifest generation still isn't ideal (parser structure -> streaming info structure -> jsx elements tree -> string) but these changes will at least improve it a bit, without major refactoring.