Fixed the issue about Validating User input in Annual tax and Exemptions

[Prateek027-S]

Issue Number 49

fixes #

ISSUE #49 #### Describe the changes you've made I used Regular Expressions to validate the user input. If the user enters invalid input or no input then an error message-box shows up. #### Describe if there is any unusual behaviour of your code(Write `NA` if there isn't) NA #### Additional context (OPTIONAL) Add any other context or screenshots about the feature request here. #### Test plan (OPTIONAL) Testing can be done by entering any invalid input such as characters or combination of numbers and characters. Or by leaving the fields empty and clicking the Calculate button. #### Checklist

• [ ] I have added my name in the contributors list at the end of README.md file.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [x] The title of my pull request is a short description of the requested changes.
• [x] Open Source Program names(LGMSOC'21)

[QAZIMAAZARSHAD]

Resolve conflicts

[Prateek027-S]

Okay