feat(extmarks): add strict option

The strict option, when set to false, allows placing extmarks on
invalid row and column values greater than the maximum buffer row
or line column respectively.

This allows for nvim_buf_set_extmark
to be a drop-in replacement for nvim_buf_set_highlight.

src/nvim/api/keysets.lua

@@ -21,6 +21,7 @@ return {
     "virt_lines";
     "virt_lines_above";
     "virt_lines_leftcol";
+    "strict";
   };
   keymap = {
     "noremap";