Allow override Bootstrap 5 variables

Problem/Motivation

Vartheme BS5 is a base theme depends on Bootstrap 5.
Vartheme has one file for bootstrap, and there is no way to override bootstrap variables.

Proposed resolution

Add a new file called _bootstrap-variables.scss to allow override bootstrap variables.
This file will be injected in bootstrap.scss before default bootstrap variables.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-10.0.0-alpha1, vartheme_bs5-2.0.0-alpha3

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3321160 by Abdullah Yassin: Allowed override Bootstrap 5 variables