feat(api): nvim_set_hl{update:boolean} #37546

Problem: nvim_set_hl always replaces all attributes.

Solution: Add update field. When true, merge with existing
attributes instead of replacing. Unspecified attributes are preserved.
If highlight group doesn't exist, falls back to reset mode.

runtime/lua/vim/_meta/api_keysets.lua

@@ -329,6 +329,7 @@ error('Cannot require a meta file')
 --- @field fg_indexed? boolean
 --- @field bg_indexed? boolean
 --- @field force? boolean
+--- @field update? boolean
 --- @field url? string
 
 --- @class vim.api.keyset.highlight_cterm