Adding support for client-defined locale detection rules

[mesr]

• Added instance property this._localeDetectionRule with, as a default value, the previously hard-coded rule that consists in reading the value of the lang query-string parameter when present;
• Added function setLocaleDetectionRule(rule) that allows client code to replace the default locale detection rule with a custom rule;
• Moved the locale detection code into its own separate function.
• Added TS type signature for the new setLocaleDetectionRule(rule) function.

[mesr]

Oh... yes, of course. How could I have missed that.