Ensure CSSTree does not hold onto last parsed string

When done compiling, force CSSTree to parse an empty string, so
as to ensure it doesn't keep a reference to that string.

Typically, the string passed to CSSTree is a small slice of a
larger string which is a whole filter list. This means that
holding a reference to the sliced string causes the JS engine
to hold in memory to the whole filter list last parsed.

src/js/static-filtering-parser.js

@@ -908,6 +908,10 @@ export class AstFilterParser {
         this.scriptletArgListParser = new ArgListParser(',');
     }
 
+    finish() {
+        this.selectorCompiler.finish();
+    }
+
     parse(raw) {
         this.raw = raw;
         this.rawEnd = raw.length;
@@ -3229,6 +3233,14 @@ class ExtSelectorCompiler {
         this.error = undefined;
     }
 
+    // CSSTree library holds onto last string parsed, and this is problematic
+    // when the string is a slice of a huge parent string (typically a whole
+    // filter lists), it causes the huge parent string to stay in memory.
+    // Asking CSSTree to parse an empty string resolves this issue.
+    finish() {
+        cssTree.parse('');
+    }
+
     compile(raw, out, compileOptions = {}) {
         this.asProcedural = compileOptions.asProcedural === true;
 

src/js/storage.js

@@ -1102,6 +1102,7 @@ import {
     }
 
     compiler.finish(writer);
+    parser.finish();
 
     // https://github.com/uBlockOrigin/uBlock-issues/issues/1365
     //   Embed version into compiled list itself: it is encoded in as the