Pixel: update letter spacing and line height (#6487)

* Update letter spacing and line height.

* Remove line height declaration from post title.

* Use generic heading setting, and override in h6.

Co-authored-by: Madhu Dollu <madhusudhan.dollu@gmail.com>

pixel/theme.json

@@ -374,7 +374,9 @@
 			},
             "heading": {
                 "typography": {
-                    "fontWeight": "500"
+                    "fontWeight": "500",
+                    "letterSpacing": "0.05em",
+                    "lineHeight": "1.2"
                 }
             },
             "h1": {
@@ -407,7 +409,8 @@
                 "typography": {
                     "fontSize": "0.75rem",
                     "fontWeight": "400",
-                    "textTransform": "uppercase"
+                    "textTransform": "uppercase",
+                    "lineHeight": "1.6"
                 }
             },
             "link": {