Add locales.json for ilib-assemble

[seonmiJin]

Checklist

The following lists affects Pub Points on pub.dev when the package is published.

• Passed the all tests.
• Verified that the example app works.
• Executed the dart format command.
• Added the API description is added if necessary.

Description

Add a locales.json file for ilib-assemble.
The ilib-assemble supports two ways to specify locales:

• --locales (-l): A comma-separated list of BCP-47 locale tags.
• --localefile (-x): A JSON file containing a locales property, which is an array of BCP-47 locale tags.

locales.json that lists the supported locales so developers can use the --localefile option when running ilib-assemble.