ISO6391.validate() type predicate

[alexgleason]

Adds a type predicate to ISO6391.validate(), so you can do this:

if (ISO6391.validate(lang)) {
  lang; // type is LanguageCode
}

The current behavior without this change is like this:

if (ISO6391.validate(lang)) {
  lang; // type is string
}

This is useful for working with the LanguageCode type throughout a project, like when functions only accept a LanguageCode parameter, or a function needs to return a LanguageCode.

Currently type assertion is required: as LanguageCode. After this change it would not be.

For example:

function iso6393To6391(iso6393: string): LanguageCode | undefined {
  const locale = new Intl.Locale(iso6393);

  if (ISO6391.validate(locale.language)) {
-   return locale.language as LanguageCode;
+   return locale.language;
  }
}

[meikidd]

@alexgleason I have published a new version v3.1.5, please kindly check.

[alexgleason]

@meikidd This is working great! See the improvement here: https://gitlab.com/soapbox-pub/ditto/-/merge_requests/637/diffs Thank you.