[LaunchPad] support L10N pluralization

[jasonLaster]

This issue is a copy of https://github.com/devtools-html/debugger.html/issues/1142.

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There are strings, which we would like to pluralize in the debugger.

A good place to start is to pluralize the source.search results summary.

here are some relevant links

• https://dxr.mozilla.org/mozilla-central/source/toolkit/locales/en-US/chrome/global/intl.properties
• http://searchfox.org/mozilla-central/source/devtools/shared/plural-form.js
• http://searchfox.org/mozilla-central/source/devtools/client/webconsole/new-console-output/components/message-repeat.js#31
• http://searchfox.org/mozilla-central/source/devtools/client/locales/en-US/webconsole.properties#131

[jasonLaster]

https://github.com/juliandescottes/sprintf.js