doc: claim ABI version for Electron v12

PR-URL: #34816
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/abi_version_registry.json

@@ -1,5 +1,6 @@
 {
   "NODE_MODULE_VERSION": [
+    { "modules": 87, "runtime": "electron", "variant": "electron",             "versions": "12" },
     { "modules": 86, "runtime": "node",     "variant": "v8_8.4",               "versions": "15.0.0-pre" },
     { "modules": 85, "runtime": "electron", "variant": "electron",             "versions": "11" },
     { "modules": 84, "runtime": "node",     "variant": "v8_8.3",               "versions": "15.0.0-pre" },