[b-form-checkbox] Add option to modify checkbox size

[hoanghaithanh]

Problem

There is no option to customize the checkbox size. The default size is quite small, it may hurt the user experience.

Preferable solution

It would be great if it has property that set the size of the checkbox when init b-form-checkbox.

[tmorehouse]

This is an upstream issue with Bootstrap v4.3 SCSS (sizing for custom checkboxes/radios/file-input).

It is being addressed in Bootstrap v5.0 (by switching the SCSS sizes of the custom control indicators to em units rather than explicit rem units, so that they will scale with the base font size of the input.

twbs/bootstrap#27803
twbs/bootstrap#27958

Unfortunately they are not backporting the update to the v4.x branch twbs/bootstrap#28450