feat(hl/editor): add Dimmed hl

[comfysage]

i dont know if we want to link related highlight groups to this new Dimmed group or keep the original fg = C.overlay1.
some hls this could work for:

• NeoTreeDimText
• Conceal
• NeoTestIndent (which should probably be using the same as IblIndent (suface0)
• NeoTestExpandMarker
• SagaCount
• DropBarIconUISeparator

neovim/neovim#39505

[vollowx]

if we want to link related highlight groups to this new Dimmed group

Considering the purpose of Dimmed, I think doing so would be nice for

• NeoTreeDimText
• NeoTestExpandMarker
• SagaCount
• DropBarIconUISeparator

(from what you've listed).

However, these hls might get linked within the plugins, and this isn't a very blocking problem, so it should be fine to merge this MR before doing all that.