Add borderless (tertiary) loading button variant

Problem

When the tertiary button without a border is exchanged for a loading button with a border the visual result can appear like an error in the UI since the perceived spacing in relation to other elements is changed.

Solution

Create a tertiary loading button variant where:

• Primary and secondary variants still use the current loading button.
• Tertiary variants use a new borderless loading button.
• Only have a neutral dark and light option for spinner color.

Before	After

Other considerations:

• The loading button text label should have at least a 3:1 contrast ratio on the background. This should be evaluated for variants with and without backgrounds. Saving this for a future update #1044 (moved).