feat(marks): add conceal_lines to nvim_buf_set_extmark()

Implement an extmark property that conceals lines vertically.

runtime/lua/vim/_meta/api.lua

@@ -685,6 +685,10 @@ function vim.api.nvim_buf_line_count(buffer) end
 ---     When a character is supplied it is used as `:syn-cchar`.
 ---     "hl_group" is used as highlight for the cchar if provided,
 ---     otherwise it defaults to `hl-Conceal`.
+--- - conceal_lines: string which should be empty. When
+---     provided, lines in the range are not drawn at all
+---     (according to 'conceallevel'); the next unconcealed line
+---     is drawn instead.
 --- - spell: boolean indicating that spell checking should be
 ---     performed within this extmark
 --- - ui_watched: boolean that indicates the mark should be drawn

runtime/lua/vim/_meta/api_keysets.lua

@@ -266,6 +266,7 @@ error('Cannot require a meta file')
 --- @field line_hl_group? integer|string
 --- @field cursorline_hl_group? integer|string
 --- @field conceal? string
+--- @field conceal_lines? string
 --- @field spell? boolean
 --- @field ui_watched? boolean
 --- @field undo_restore? boolean