Add diamond icon

[kelsey-taylor]

Description of change

Adds a diamond icon for use as a marker.

Link to demonstration

Link to style with icons

Closes #533.

For author

• [x] Cite changes under the HEAD tag in CHANGELOG.md
• [x] PR contains only one 15x15 icon
• [x] Changes fall in accordance to the Maki guidelines
  • [x] Icon geometry is aligned to the pixel grid
  • [x] Icon stays within a 2px trim area
  • [x] Icon uses common geometric building blocks used throughout Maki
  • [x] Icon corners are rounded in either full or half pixel increments
  • [x] Any strokes use 1px
• [x] Upload the new icon to the Maki icon editor
  • [x] Compare with existing icons
  • [x] Test that stroke/backgrounds/padding work visually

For reviewer

• [ ] Review the interactive map provided to confirm legibility and harmony with existing icons
• [ ] Review each icon variation in an editor, turn on grids and outlines and confirm pixel alignment is maintained to the nearest or half pixel
• [ ] Confirm file requirements stated in Maki guidelines are met
• [ ] Confirm all other changes fall in accordance to the Maki guidelines