Add formatter cache

[pwolaq]

This PR resolves #1841.

Few considerations:

• maybe we should export createCachedFormatter as a utility function so that others may reuse it when creating custom formatters?
• I had no idea how to test that functionality apart from somehow stubbing Intl constructors and verifying that they are not called multiple times - I can try to add this test if you would like to have it
• as cache keys are always strings I decided to use object as caching structure instead of Map (which might cause compatibility issues in older browsers)

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

Coverage increased (+0.1%) to 91.151% when pulling 48fcdef on pwolaq:formatter-cache into 69313fc on i18next:master.

[adrai]

As alternative to the exposure of the createCachedFormatter function, we could extend the add function... something like add(name, fc, getFormatterFc) and cache that automatically... just an idea before going to bed 😉

[adrai]

As alternative to the exposure of the createCachedFormatter function, we could extend the add function... something like add(name, fc, getFormatterFc) and cache that automatically... just an idea before going to bed 😉

what do you think about something like this?

add(name, fc) {
  this.formats[name.toLowerCase().trim()] = fc;
},

addCached(name, fc) {
  this.formats[name.toLowerCase().trim()] = createCachedFormatter(fc)
}


i18next.services.formatter.addCached('my-format', (lng, options) => {
  const formatter = new Intl.NumberFormat(lng, options);
  return (val) => formatter.format(val);
})

[adrai]

landed in v21.10.0