Bug 1854112 - Make HTMLSelectElement showPicker method Nightly-only. r=emilio

When I landed this initially, it looked to me like the spec PR was about to land, but now there is an ongoing discussion about the behavior in uncomposed documents and even concerns with the previous <input> showPicker method.

Differential Revision: https://phabricator.services.mozilla.com/D192747

browser/base/content/test/forms/browser.toml

@@ -1,5 +1,5 @@
 [DEFAULT]
-prefs = ["gfx.font_loader.delay=0"]
+prefs = ["gfx.font_loader.delay=0", "dom.select.showPicker.enabled=true"]
 support-files = ["head.js"]
 
 ["browser_selectpopup.js"]

modules/libpref/init/StaticPrefList.yaml

@@ -3776,7 +3776,7 @@
 
 - name: dom.select.showPicker.enabled
   type: bool
-  value: true
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 - name: dom.separate_event_queue_for_post_message.enabled

testing/web-platform/meta/html/semantics/forms/the-select-element/__dir__.ini

@@ -0,0 +1 @@
+prefs: [dom.select.showPicker.enabled:true]