Make the WebGL terminal renderer the default

[Tyriar]

Part of #106202

We should also improve the fallback logic to do this:

• Try webgl
• If it threw an exception, try canvas
• If frame rendering averages > 50ms (existing detection), use DOM

We will keep the canvas renderer around for now mainly because Safari doesn't support webgl2 and can evaluate whether we want to remove it again at a later date.

[connor4312]

It looks like canvas is still the default for me -- the xterm-text-layer is present unless I explicitly enable experimentalWebgl. I don't see any notifications of errors that could be causing it.