Release js_of_ocaml 6.4

[hhugo]

With OCaml 5.5 approaching, we need a new jsoo/wasmoo release.

Track issue and PR we want/need for the next release:

• Switch to dune 3.23 #2231 (optional)
• Lib: add Promise + Fetch bindings (#2031, #596) #2239 (as a natural follow up from Dom updates #2221)
• Update to Oxcaml 5.2.0minus31 #2225 (Maybe)
• Add Intl.RelativeTimeFormat #2070 (if possible)
• Lib: fix onbeforeunload handler breaking navigation (#1436)  #2159 (seems unlikely, we need feedback from Janestreet)
• Wasm runtime: small fixes #2240 (small fixes)

[hhugo]

@vouillon, what do you think ?