Correct the regular expression for explicit plural forms

/\d+=/ will match a12=abc. That is wrong. The plural form must
start with digits. Changing it to /^\d+=/

Also avoid Regex instance creation in mediawiki.language.js, use
literal regex.

Follow up: I82b9067 and I50eb0c6
Bug: 51284

Change-Id: I70ad3f99ad0a2600773c005d9e9cc8238e6e6d09
Merge "Correct the regular expression for explicit plural forms"