Properly serialize CSS combinators according to position in selector

Related issue:
https://github.com/uBlockOrigin/uBlock-issues/issues/2778

Regression from:
bb41d9594f

The regression occurred because the modified code made the assumption
that a leading combinator would never be preceded by whitespace, while
the parser didn't prevent this.

The parser has been fixed to ensure there is never a leading
whitespace in a selector.

src/js/static-filtering-parser.js

@@ -3364,7 +3364,7 @@ class ExtSelectorCompiler {
             out.push(`.${data.name}`);
             break;
         case 'Combinator':
-            out.push(data.name === ' ' ? ' ' : ` ${data.name} `);
+            out.push(data.name);
             break;
         case 'Identifier':
             if ( this.reInvalidIdentifier.test(data.name) ) { return; }
@@ -3436,6 +3436,28 @@ class ExtSelectorCompiler {
         return out.join('');
     }
 
+    astAppendPart(part, out) {
+        const { data } = part;
+        switch ( data.type ) {
+            case 'Combinator': {
+                const s = this.astSerializePart(part);
+                if ( s === undefined ) { return false; }
+                if ( out.length === 0 ) {
+                    if ( s !== ' ' ) {
+                        out.push(s, ' ');
+                    }
+                } else {
+                    out.push(' ');
+                    if ( s !== ' ' ) {
+                        out.push(s, ' ');
+                    }
+                }
+                break;
+            }
+        }
+        return true;
+    }
+
     astSerialize(parts, plainCSS = true) {
         const out = [];
         for ( const part of parts ) {
@@ -3443,7 +3465,6 @@ class ExtSelectorCompiler {
             switch ( data.type ) {
             case 'AttributeSelector':
             case 'ClassSelector':
-            case 'Combinator':
             case 'Identifier':
             case 'IdSelector':
             case 'Nth':
@@ -3455,6 +3476,9 @@ class ExtSelectorCompiler {
                 out.push(s);
                 break;
             }
+            case 'Combinator':
+                if ( this.astAppendPart(part, out) === false ) { return; }
+                break;
             case 'Raw':
                 if ( plainCSS ) { return; }
                 out.push(this.astSerializePart(part));
@@ -3499,7 +3523,6 @@ class ExtSelectorCompiler {
             }
             case 'AttributeSelector':
             case 'ClassSelector':
-            case 'Combinator':
             case 'IdSelector':
             case 'PseudoClassSelector':
             case 'PseudoElementSelector':
@@ -3509,6 +3532,10 @@ class ExtSelectorCompiler {
                 prelude.push(component);
                 break;
             }
+            case 'Combinator': {
+                if ( this.astAppendPart(part, prelude) === false ) { return; }
+                break;
+            }
             case 'ProceduralSelector': {
                 if ( prelude.length !== 0 ) {
                     let spath = prelude.join('');