make some refactor on symbols

[yousef-nageh]

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• I have made myself familiar with the CaTeX
  contributing guide.
• I added a PR description.
• I linked all related issues and PRs I could find (no links if there are none).
• If this PR changes anything about the main math_keyboard or example package
  (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change
  on its own is not worth an update).
• If this PR includes a notable change in the math_keyboard package, I updated the version
  according to Dart's semantic versioning.
• If there is new functionality in code, I added tests covering all my additions.
• All required checks pass.

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