Add indentation requirement for line continuation

Related commit:
- https://github.com/gorhill/uBlock/commit/703c525b01aa

This adds an indentation requirement for line
continuation to take place. The conditions are now
as follow:
- Current line ends with ` \`: ASCII space + backslash
- Next line starts with `    `: four ASCII spaces

src/js/codemirror/ubo-static-filtering.js

@@ -121,6 +121,7 @@ CodeMirror.defineMode("ubo-static-filtering", function() {
                 if ( line.endsWith(' \\') ) {
                     do {
                         line = stream.lookAhead(lines.length);
+                        if ( line.startsWith('    ') === false ) { break; }
                         lines.push(line);
                     } while ( line.endsWith(' \\') );
                 }

src/js/storage.js

@@ -808,6 +808,7 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
         if ( line.length === 0 ) { continue; }
 
         while ( line.endsWith(' \\') ) {
+            if ( lineIter.peek(4) !== '    ' ) { break; }
             line = line.slice(0, -2).trim() + lineIter.next().trim();
         }
 

src/js/utils.js

@@ -242,6 +242,10 @@
         this.offset = lineEnd + 1;
         return line;
     }
+    peek(n) {
+        const offset = this.offset;
+        return this.text.slice(offset, offset + n);
+    }
     charCodeAt(offset) {
         return this.text.charCodeAt(this.offset + offset);
     }