Add flag parsing and input validation methods

[catatsuy]

This pull request introduces significant refactoring to the CLI struct and its methods in the cli/cli.go file, aiming to improve the readability and maintainability of the code. The changes mainly involve moving flag parsing and input validation logic from the Run method into separate methods, and changing local variables in the Run method to struct fields. Additionally, the test TestRun_failToProvideStdin in cli/cli_test.go has been modified to accommodate these changes.

Here's a summary of the most important changes:

Refactoring in cli/cli.go:

• CLI struct: Added new fields filePath, replaceExpr, isOverwrite, and filters to store the command-line flags' values.
• Run method: Moved the flag parsing logic into a new method parseFlags and the input validation logic into a new method validateInput. Replaced the local variables filePath, replaceExpr, isOverwrite, and filters with the newly added struct fields. [1] [2] [3] [4] [5]
• New parseFlags method: This method handles the parsing of command-line flags and assigns the values to the corresponding struct fields.
• New validateInput method: This method validates the input based on the parsed flags and handles error messages.

Changes in cli/cli_test.go:

• TestRun_failToProvideStdin test: Updated the test to handle the changes in the Run method, and added several new test cases to improve test coverage.