feat: establish universal inverse color highlight groups

[adriantrunzo]

Extracted from #336. These changes standardize highlight groups for inverse colors. Of note:

• The foreground (text) color for each inverse group is s:bg instead of s:none to ensure sufficient contrast. The DraculaRedInverse group used for error messages is no longer white text on a red background.
• The DraculaSearch is functionally redundant with DraculaGreenInverse, but retained as a semantic highlight group for backwards compatibility. DraculaSearch is now just a linked group.