Add global configuration and methods to enable the use of `calc` by default in numeric operations

[SalimBensiali]

Looking at https://github.com/twbs/bootstrap/blob/72d3b6efc4b92e83a4abca6f5bc0cd7e6fd25931/scss/_variables.scss#L925 while add is using calc by default (See https://github.com/twbs/bootstrap/blob/main/scss/_functions.scss#L222), if $input-padding-y is overridden to use calc (say calc(2 * 14px)), then the entire assignment will fail with an error such as

Undefined operation "calc(2 * 14px) * 2".

because the outer multiplication by 2 is not using calc itself.

Could we add a global and overridable SCSS variable ($return-calc) and introduce a multiply function that would use that variable to decide whether to return calc or not? The existing add could also benefit from that global $return-calc variable.

The original assignment would then turn change to the below and would no longer fail.

add(multiply($input-line-height, 1em), multiply($input-padding-y, 2)) !default;

I would be happy to supply a PR for this if you see value in it.

Cheers