Add core module

[fityannugroho]

PR Checklist

Please check if your PR fulfills the following requirements:

Put [x] to check

• I have read the documentation.
• I have read and followed the Contributing Guidelines.
• I have included a pull request description of my changes.
• I have included the necessary changes to the documentation.
• I have added tests to cover my changes.

PR Type

What kind of change does this PR introduce?

Please check any kind of changes that applies to this PR using [x]

• Bug fix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• ..... (describe the other type)

What is the current behavior?

Please describe the current behavior that you are modifying, or link to a relevant issue.

Issue Number: N/A

Currently, this package can be run only in Node.js environment. It doesn't works well for another environments including browsers and some edge runtimes which is doesn't support file system module (see fityannugroho/wisely-web#3).

What is the new behavior?

Create core module (wisely/core) that doesn't use built-in charsets, so it doesn't need file system module.

Other information

None