feat: Add checkBase64 util function

[klmhyeonwoo]

Description

• Base64 문자열이 들어왔는지 확인하는 검증 유틸 함수를 추가했어요

closes: #17

[Copilot]

Pull Request Overview

This PR adds a utility function to validate Base64 encoded strings, expanding the validation utilities available in the package.

• Implements checkBase64 function with regex-based validation
• Adds comprehensive test coverage for valid and invalid Base64 strings
• Updates package exports and documentation

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
package/validationUtil/index.ts	Exports the new checkBase64 function
package/validationUtil/checkBase64/index.ts	Implements Base64 validation using regex pattern
package/validationUtil/checkBase64/index.test.ts	Provides test cases for valid and invalid Base64 strings
README.md	Documents the new checkBase64 function with usage example

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[klmhyeonwoo]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	63.57%	96 / 151
🔵	Statements	63.57%	96 / 151
🔵	Functions	93.75%	15 / 16
🔵	Branches	96.07%	49 / 51

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
package/validationUtil/checkBase64/index.ts	100%	100%	100%	100%

Generated in workflow #42 for commit adfb5ab by the Vitest Coverage Report Action

[prgmr99]

고생하셨습니다~~~👍