Merge branch 'feature/3-adding-more-constants' into 'master'

Operating System Constants, Variants, More missing constants

Closes #3, #5 , #6, #7

## Introduction

Operating System Constants, Variants, More missing constants

Note: [Template Reference](https://hackmd.io/@akarimevatix/H1Qsmq23w)

## Worked on Items

- [x] #3 - Adding more consts
- [x] #5 - **Operating System Constants, Variants**, operating system quicker osconsts.IsWindws/IsLinux/NewLine/PathSeparator
- [x] #8 - tests core skip feature
- [x] #6 - changed package name to `core` changed respective readme files.
- [x] #7 - filemodes and references integrated.

## TODOs

- None

## References

* [chmod Man Page with examples and calculator - Linux - SS64.com](https://ss64.com/bash/chmod.html)
* [unix - How does the go language os.FileMode function converts permissions from integers/octal/??? before setting the flags? - Stack Overflow](https://stackoverflow.com/questions/48123541/how-does-the-go-language-os-filemode-function-convert-permissions-from-integers)
* [The Go Playground](https://play.golang.org/p/Qq_rKl_pAqe)
* [unix - How does the go language os.FileMode function converts permissions from integers/octal/??? before setting the flags? - Stack Overflow](https://stackoverflow.com/questions/48123541/how-does-the-go-language-os-filemode-function-convert-permissions-from-integers)
* [The Go Playground](https://play.golang.org/p/dX-wsvJmFie)
* [The Go Playground](https://play.golang.org/p/Qq_rKl_pAqe)

## Notes

**Based on the functionalities added, package name needs to be changed to `core` instead of constants only** FYI: @nafrose

## Merge Request Ordering

FYI: @nafrose

Ordering: None

See merge request evatix-go/constants!5