Refactor constants to a separate file

[yash-chowdhary]

What is the purpose of this pull request?

• [x] Other: Refactoring constants into a separate file.

Fixes #597

What is the rationale for this request?
Currently, there are multiple occurrences of the same constant across multiple files e.g. LAYOUT_FOLDER_PATH. This PR addresses this issue by keeping all constants in a single file.

What changes did you make?
Moved all constants in functional code to a new file - src/constants.js.

Provide some example code that this change will affect:

module.exports = {
  // index.js
  ACCEPTED_COMMANDS: ['init', 'build', 'serve', 'deploy'],
  ACCEPTED_COMMANDS_ALIAS: ['i', 'b', 's', 'd'],

  // src/Page.js
  FOOTERS_FOLDER_PATH: '_markbind/footers',
  HEAD_FOLDER_PATH: '_markbind/head',
  ...
  ...
}

Proposed commit message: (wrap lines at 72 characters)
Refactor constants to a separate file

[acjh]

Hi Yash, thanks for your PR.

I've left a comment about separating constants in MarkBind core library from MarkBind CLI, and a few comments about consistency.