Bug 1748798 - Add 'Firefox 100 User-Agent String' to the Nightly Experiments settings. r=jaws,flod

The Nightly experiment's description:

> Firefox 100 User-Agent String
>
> Make Nightly send websites a User-Agent string that pretends to be Firefox
> version 100. Use this setting to test whether websites will break when
> Nightly hits a three-digit version number. The real Firefox 100 is scheduled
> to be released in May 2022, so start testing your websites now!

Firefox's User-Agent string says "Firefox/100.0" in both release and pre-release channels. Firefox 100's release date will be 2022-05-03. The Nightly 100 development cycle will begin 2022-03-08.

Chrome has a similar chrome://flags/#force-major-version-to-100 flag for testing a Chrome 100 UA.

Differential Revision: https://phabricator.services.mozilla.com/D135316

toolkit/components/featuregates/Features.toml

@@ -132,3 +132,13 @@ type = "boolean"
 bug-numbers = [1673971]
 is-public = true
 default-value = false
+
+[firefox-100]
+title = "experimental-features-firefox-100"
+description = "experimental-features-firefox-100-description"
+restart-required = false
+preference = "general.useragent.forceVersion100"
+type = "boolean"
+bug-numbers = [1748798]
+is-public = true
+default-value = false

toolkit/locales/en-US/toolkit/featuregates/features.ftl

@@ -77,3 +77,8 @@ experimental-features-js-warp-description = Enable Warp, a project to improve Ja
 experimental-features-ime-search =
     .label = Address Bar: show results during IME composition
 experimental-features-ime-search-description = An IME (Input Method Editor) is a tool that allows you to enter complex symbols, such as those used in East Asian or Indic written languages, using a standard keyboard. Enabling this experiment will keep the address bar panel open, showing search results and suggestions, while using IME to input text. Note that the IME might display a panel that covers the address bar results, therefore this preference is only suggested for IME not using this type of panel.
+
+# Firefox 100
+experimental-features-firefox-100 =
+    .label = { -brand-product-name } 100 User-Agent String
+experimental-features-firefox-100-description = Make { -brand-short-name } send websites a User-Agent string that pretends to be { -brand-product-name } version 100. Use this setting to test whether websites will break when { -brand-short-name } hits a three-digit version number. The real { -brand-product-name } 100 is scheduled to be released in May 2022, so start testing your websites now!