feat(BTable): add noSortReset option

[dnewkerk]

Describe the PR

Implements the noSortReset option for BTable. When enabled, sorting by a field only toggles between ascending and descending, and the sortBy value is not reset.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix 🐛 - fix(...)
• Feature - feat(...)
• ARIA accessibility - fix(...)
• Documentation update - docs(...)
• Other (please describe)

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[VividLemon]

This is actually something that I had added in my current branch. Which contains a few other changes to Table. So, I'm going to prioritize that over this. Thanks though

[VividLemon]

Actually, I'm going to keep this around. I may have misinterpreted what this prop actually does. When 0.17 is added, I may need you to check this PR again

[VividLemon]

Actually, the functionality from bootstrap-vue is close to what I have done already. However, it's not perfect. It's going to be a deprecated prop.

In bootstrap-vue, the no-sort-reset would only modify the sort state when you click on a column that is not sortable. Meaning that if the entire table is sortable, one could never "unsort" the items.

Instead, the functionality has changed. prop mustSort is going to do this, where if it's true, you could never (from the tables pov) unsort a column. If it's false, you can click to 'asc' -> 'desc' -> null (unsorted). In addition, one could make it an array (since it's multisortable) where you could say some fields are mustsort, while others aren't

(0.17 use prop :must-sort)