Make the next version emscripten 4.0.0! (emscripten-core#23235)

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
- Lots of old polyfills finally removed

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!

Author: sbc100

ChangeLog.md

@@ -18,8 +18,8 @@ to browse the changes between the tags.
 
 See docs/process.md for more on how version tagging works.
 
-3.1.75 (in development)
------------------------
+4.0.0 (in development)
+----------------------
 - compiler-rt, libcxx, and libcxxabi were updated to LLVM 19.1.4. (#22937 and
   #22994)
 - The Wasm nontrapping-fptoint feature has been enabled by default. clang will

emscripten-version.txt

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