Add a new stage 1 proposal for 128-bit arithemetic to CG-08-13.md (#1639

)

@jameysharp and I would like to give a presentation about a possible new
stage 1 proposal for the CG at this meeting to see what others' thoughts
are on it.

main/2024/CG-08-13.md

@@ -15,6 +15,7 @@ No registration is required for VC meetings. The meeting is open to CG members o
 1. Opening
 1. Proposals and discussions
    1. Update on the ESM integration (Guy Bedford, 25 minutes)
+   2. New stage 1 proposal: [128-bit arithmetic](https://github.com/WebAssembly/design/issues/1522) (Alex Crichton / Jamey Sharp, 35 minutes)
 1. Closure
 
 ## Agenda items for future meetings