Making progress bar styleable with scss variables

[Larsmyrup]

Description

Made the progress bar background styleable using scss variables. Also made the variable used change between toast type, in a similar manner to the toast background and color.

Related Issue

https://github.com/Maronato/vue-toastification/issues/358

Screenshots or GIFs (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] I have read the CONTRIBUTING document.
• [x] All new and existing tests passed. Don't think there currently are any established way of testing the SCSS variables. Let me know if you have any ideas of how to, and i'll look into implementing it.