Correctly escape CSS before injecting into the pane

Example CSS that broke injection before, but not
now (from Apple Dictionaries CSS):

```css
span.hsb:before
{
    /* '•' */
    content: '\2022';
    font-size: 80%;
    vertical-align: 3%;
}
```

AlfredExtraPane.xcodeproj/project.pbxproj

@@ -413,7 +413,7 @@
 					"@executable_path/../Frameworks",
 				);
 				MACOSX_DEPLOYMENT_TARGET = 11.0;
-				MARKETING_VERSION = 0.2.5;
+				MARKETING_VERSION = 0.2.6;
 				ONLY_ACTIVE_ARCH = NO;
 				OTHER_CODE_SIGN_FLAGS = "--deep";
 				PRODUCT_BUNDLE_IDENTIFIER = mr.pennyworth.AlfredExtraPane;
@@ -434,7 +434,7 @@
 					"@executable_path/../Frameworks",
 				);
 				MACOSX_DEPLOYMENT_TARGET = 11.0;
-				MARKETING_VERSION = 0.2.5;
+				MARKETING_VERSION = 0.2.6;
 				ONLY_ACTIVE_ARCH = NO;
 				OTHER_CODE_SIGN_FLAGS = "--deep";
 				PRODUCT_BUNDLE_IDENTIFIER = mr.pennyworth.AlfredExtraPane;

AlfredExtraPane/WebViewInjection.swift

@@ -9,9 +9,10 @@ private let cssInjectedMessageName = "cssInjected"
 /// The only way is to inject a <style> tag into the document by executing
 /// JS in the webview.
 private func injecterJS(_ cssString: String) -> String {
-  """
+  let escapedCssString = cssString.replacingOccurrences(of: "\\", with: "\\\\")
+  return """
   var style = document.createElement('style');
-  style.innerHTML = `\(cssString)`;
+  style.innerHTML = `\(escapedCssString)`;
   document.head.appendChild(style);
   window.webkit.messageHandlers.\(cssInjectedMessageName).postMessage('done');
   """