Make the next version emscripten 4.0.0

We have a whole bunch of significant change in the next release:

- 3 new wasm features enabled by default
- Use of async/await internally and in MODULARIZE mode
- LLVM libraries updated for v19.1

Also, 74 revisions the 3.1 release is gettting pretty long in the tooth.

Plus it will be the first release of the new year!

ChangeLog.md

@@ -22,6 +22,8 @@ See docs/process.md for more on how version tagging works.
 -----------------------
 - compiler-rt, libcxx, and libcxxabi were updated to LLVM 19.1.4. (#22937 and
   #22994)
+4.0.0 (in development)
+----------------------
 - The Wasm nontrapping-fptoint feature has been enabled by default. clang will
   generate nontrapping (saturating) float-to-int conversion instructions for
   C typecasts. This should have no effect on programs that do not have

emscripten-version.txt

@@ -1 +1 @@
-3.1.75-git
+4.0.0-git