fix: preserve loose list spacing in MarkdownRenderer

[gaoflow]

Problem

MarkdownRenderer.render_list() always joins rendered list items with
"", ignoring the element.tight attribute. This means a loose
list (items separated by blank lines, tight=False) is re-emitted as
a tight list, silently changing the document's semantics.

The difference matters: a tight list renders list content as bare text,
while a loose list wraps each item in <p> tags.

import marko
from marko.md_renderer import MarkdownRenderer

md = marko.Markdown(renderer=MarkdownRenderer)

# Before the fix:
md("- item1\n\n- item2\n")
# Returns: '- item1\n- item2\n'  ← blank line lost, semantics changed

Converting the re-rendered output back to HTML gives different HTML:

Input	HTML
- item1\n\n- item2\n (loose)	<ul><li><p>item1</p></li><li><p>item2</p></li></ul>
- item1\n- item2\n (tight)	<ul><li>item1</li><li>item2</li></ul>

Fix

When element.tight is False, join the rendered items with "\n"
(a blank line) instead of "".

Tests

Added test_markdown_renderer_loose_list covering loose unordered
lists, 3-item loose lists, and loose ordered lists — verifying both
exact text round-trip and HTML semantic equivalence.

All 1397 tests pass.