Support multiple charsets and custom charset

[fityannugroho]

PR Checklist

Please check if your PR fulfills the following requirements:

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• 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
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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, wisely can only using one built-in charset.

What is the new behavior?

• Add mergeCharSets() function to merge multiple charsets (both built-in and custom charset).
• Rename wisely()'s options.charSet into options.charSets that receive an array and can contain both built-in and custom charset.
• Validate built-in charset and custom charset with isCharSetValid().

Other information

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