feat(date-picker): add component tokens

[anveshmekala]

Related Issue: #

Summary

Adds following tokens for calcite-date-picker component.

 @prop --calcite-date-picker-border-color: Specifies the border color of the component.
 @prop --calcite-date-picker-border-radius: Specifies the border radius of the component.
 @prop --calcite-date-picker-day-background-color-hover: Specifies the background color of the day when hovered.
 @prop --calcite-date-picker-day-background-color-selected: Specifies the background color of the day when selected.
 @prop --calcite-date-picker-day-background-color: Specifies the background color of the day.
 @prop --calcite-date-picker-day-corner-radius: Specifies the corner radius of day.
 @prop --calcite-date-picker-day-font-size: Specifies the font size of the day.
 @prop --calcite-date-picker-day-range-background-color-hover: Specifies the background color of day range when hovered.
 @prop --calcite-date-picker-day-range-background-color-selected: Specifies the background color of day range when selected.
 @prop --calcite-date-picker-day-text-color-hover: Specifies the text color of day when hovered.
 @prop --calcite-date-picker-day-text-color: Specifies the text color of day.
 @prop --calcite-date-picker-day-text-selected: Specifies the text color of day when selected.
 @prop --calcite-date-picker-week-headers-border-color: Specifies the border color of week headers.
 @prop --calcite-date-picker-week-headers-text-color: Specifies the text color of week headers.

[alisonailea]

Getting there. Just a few things.

• Remove "internal" from all component token names that are
• • Background color
• • color
• • border color
• • shadow (box-shadow)
• • icon color (fill/stroke)
• • corner radius (border-radius)
• Make sure none of the above tokens are assigned values like on date-picker-day.scss:161
• add some docs to date-picker-day.scss.