Expands 'rounded' classes

Created by: Parth-Gupta10

This PR adds first 2 features proposed in issue #31784 (closed) .

The following changes were made in scss/_utilities.scss -

1. Classes rounded-top-left, rounded-top-right, rounded-bottom-left, rounded-bottom-right were added to implement rounding of 1 corner only.
2. $spacers was added in values of rounded class to implement size increments for rounding more akin to the margin/padding sizes (0-5).