vim-patch:9.2.1036: syntax highlighting is slower than necessary (#41644)

Problem:  While advancing the syntax highlighting, store_current_state()
          calls syn_stack_find_entry() about once per parsed line, and
          that function rescans the state-cache list from its head every
          time. With the list capped at 1000 entries this linear rescan
          dominates the cost of highlighting a large file.
Solution: Keep a cached position to the entry last located in the list
          and resume the scan from it when it is at or before the wanted
          line, advancing the finger as new states are stored
          (Julien Voisin).

The list is sorted by line number, so when the remembered entry is at or
before the wanted line the answer can only follow it, never precede it;
resuming from there returns the same entry as a scan from the head.  The
"at or before" guard keeps this correct for any lookup order: a backward
or random-access lookup whose remembered entry is past the wanted line
falls back to a full scan.  The pointer is cleared whenever an entry is
freed, the array is reallocated or the block is freed, so it cannot
dangle.

Parsing the syntax of a 20000-line C file is about a third faster, a
5000-line file about a quarter faster, benchmarked via something like
this:

```
call synID(1, 1, 1)              " warm-up
let s = reltime()
for l in range(1, line('$')) | call synID(l, 1, 1) | endfor
call writefile([reltimefloat(reltime(s))], $T)
```

closes: vim/vim#21166

https://github.com/vim/vim/commit/f1b454912996d6417acd0d738de0eb7b60902c56

Co-authored-by: Julien Voisin <julien.voisin@dustri.org>

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