feat(decoration): allow conceal_char to be a composing char

decor->text.str pointer must go. This removes it for conceal char,
in preparation for a larger PR which will also handle the sign case.

By actually allowing composing chars for a conceal chars, this
becomes a feature and not just a refactor, as a bonus.

runtime/lua/vim/_meta/api.lua

@@ -80,6 +80,9 @@ function vim.api.nvim__id_float(flt) end
 function vim.api.nvim__inspect_cell(grid, row, col) end
 
 --- @private
+--- For testing. The condition in schar_cache_clear_if_full is hard to reach,
+--- so this function can be used to force a cache clear in a test.
+---
 function vim.api.nvim__invalidate_glyph_cache() end
 
 --- @private