FIx display of readme/changelog/license in Package Manager (#16829)

### What does it do?
- **Backend:** Removes `htmlspecialchars()` before Parsedown in
`GetAttribute` for readme/changelog/license. Markdown is parsed from raw
text; empty attributes are handled with `$data ?? ''`.
- **Frontend:** For the readme, changelog, and license tabs in Package
Manager, the template uses raw substitution (`{[values.id]}`) so HTML
from Parsedown is rendered without escaping.
- **CSS:** Adds Markdown styles under `.meta-wrapper`: `pre`, `code`,
`pre code`, and `p` (code blocks, inline code, paragraph spacing).
Values use `rem` units.

### Why is it needed?
Readme content during package installation was broken: paragraphs ran
together, backticks and code blocks were not styled, and HTML was shown
as literal text. This was caused by calling `htmlspecialchars()` before
Parsedown and by escaping the resulting HTML in the template.

### How to test
1. Use a package whose readme.txt has paragraphs, `` `inline code` ``, a
fenced code block, and HTML (e.g. `<br/>`).
2. Go to Extras → Install, choose the package, and open the Readme tab.
3. Confirm: paragraphs are spaced, inline and block code are visually
distinct, and HTML is rendered instead of shown as text.

### Related issue(s)/PR(s)
Resolves #16206

Author: Ibochkarev

core/src/Revolution/Processors/Workspace/Packages/GetAttribute.php

@@ -1,4 +1,5 @@
 <?php
+
 /*
  * This file is part of MODX Revolution.
  *
@@ -77,8 +78,10 @@ public function process()
         $parseDown->setSafeMode(true);
         foreach ($attributesToGet as $attribute) {
             $data = $this->transport->getAttribute($attribute);
-            $attributes[$attribute] = in_array($attribute,
-                ['changelog', 'license', 'readme']) ? $parseDown->text(htmlspecialchars($data)) : $data;
+            $attributes[$attribute] = in_array(
+                $attribute,
+                ['changelog', 'license', 'readme']
+            ) ? $parseDown->text($data ?? '') : $data;
 
             /* if setup options, include setup file */
             if ($attribute === 'setup-options') {