Support meta chars for StartWith and EndWith cops

[koic]

This PR supports regexp metacharacter ^ for Performance/StartWith cop and regexp metacharacter $ for Performance/EndWith cop.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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