Handle conversion of plural topic names to singular form.
StaticMethod singularForm ($web,$pluralForm) -> $singularForm
Try to singularise plural topic name.
$web - the web the topic must be in
$pluralForm - topic name
Returns undef if no singular form exists, otherwise returns the
singular form of the topic
I18N - Only apply plural processing if site language is English, or
if a built-in English-language web (Main, TWiki or Plugins). Plurals
apply to names ending in 's', where topic doesn't exist with plural
SMELL: this is highly langauge specific, and shoud be overridable
on a per-installation basis.