Add new methods for String class - ucFirst, contains and lower

[houssenedao]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/sinuoslabs/support/blob/main/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

• add new method ucfirst
• add new method conatains
• add new method lower

Does this PR introduce a breaking change?

• Yes
• No

Other information

BREAKING CHANGE:

• add new method ucfirst
• add new method conatains
• add new method lower

[houssenedao]

🎉 This PR is included in version 1.0.1 🎉

The release is available on GitHub release

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