Add sync update terminfo and docs

This adds documentation stating that Alacritty supports the synchronized
update escape, which was implemented in 9575aed.

Since tmux does check terminfo for this feature, the `Sync` capability
has also been added. Tmux's implementation can be found here:

f5b7ebc540/tty-features.c (L185)

docs/escape_support.md

@@ -99,4 +99,6 @@ brevity.
 
 ### DCS (Device Control String) - `ESC P`
 
-Alacritty does not support any DCS escapes.
+| ESCAPE    | STATUS      | NOTE                                               |
+| --------- | ----------- | -------------------------------------------------- |
+| `DCS = s` | IMPLEMENTED |                                                    |

extra/alacritty.info

@@ -107,3 +107,4 @@ alacritty+common|base fragment for alacritty,
     Ms=\E]52;%p1%s;%p2%s\007, Se=\E[0 q, Ss=\E[%p1%d q,
     hs, dsl=\E]2;\007, fsl=^G, tsl=\E]2;,
     Smulx=\E[4\:%p1%dm,
+    Sync=\EP=%p1%ds\E\\,