feat(ui): allow to set the highlight namespace per window

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

src/nvim/globals.h

@@ -96,9 +96,6 @@ EXTERN struct nvim_stats_s {
 EXTERN int Rows INIT(= DFLT_ROWS);     // nr of rows in the screen
 EXTERN int Columns INIT(= DFLT_COLS);  // nr of columns in the screen
 
-EXTERN NS ns_hl_active INIT(= 0);         // current ns that defines highlights
-EXTERN bool ns_hl_changed INIT(= false);  // highlight need update
-
 // We use 64-bit file functions here, if available.  E.g. ftello() returns
 // off_t instead of long, which helps if long is 32 bit and off_t is 64 bit.
 // We assume that when fseeko() is available then ftello() is too.