Update to PIXI v6

[arthuro555]

Pixi v6 is about to come out, two RCs have already been published. As far as I see, the changes are primarily bug fixes and they made the typescript typing better (which is good since we just moved to TS). I think this update would be interesting as

• It fixes bugs
• It has better typescript support
• It is compatible with pixi-projection unlike our current pixi version.

[4ian]

Upgrading is fine when we can, I think we'll need at least an upgrade for the BitmapText as there were bugfixes for it in the v5 branch.
We can either upgrade to latest v5 or jump the ship and upgrade to v6 if it's mostly bug fixes :)

[Bouh]

I'm for waiting for the V6.
For the bitmap extension I work with 5.3.7 on my local branch it's enough for me.