Implement main file functionality

[yehorbk]

Our application should have functionality for start up with arguments:

• -e - expression
• -f - input file (instead of expression)
• -o - output file for result (if not set - pipe result to default stdout (console))

Usage example:

• -e "5 5 +"
• -f my-input.txt
• -o result.txt
• go run ./cmd/example -e “42 1 -” -o result.txt
• go run ./cmd/example -f input.txt

Use flag library to parse command line arguments: https://pkg.go.dev/flag

Errors should be piped into stderr stream.
The input and output should be injected into ComputeHandler using Dependency Injection.