LibWeb: Correct initial values in Properties.json

- `align-items`: `normal` is the initial value in the CSS-ALIGN spec,
but `stretch` is in CSS-FLEXBOX. The FLEXBOX spec is the one we've
actually implemented elsewhere, and ALIGN adds new values with special
syntax, so it's not trivial to add it here.
- `border-spacing`: `0` is equivalent to `0px 0px` and we don't yet
parse the double-value syntax.
- `text-decoration-thickness`: Had the wrong value.

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -1,7 +1,7 @@
 {
   "align-items": {
     "inherited": false,
-    "initial": "normal",
+    "initial": "stretch",
     "valid-identifiers": [
       "center",
       "baseline",
@@ -314,7 +314,7 @@
   },
   "border-spacing": {
     "inherited": true,
-    "initial": "0px 0px",
+    "initial": "0",
     "quirks": [
       "unitless-length"
     ]
@@ -1195,7 +1195,7 @@
   },
   "text-decoration-thickness": {
     "inherited": false,
-    "initial": "none",
+    "initial": "auto",
     "valid-types": [
       "length",
       "percentage"