Improve Markdown handling

[philippjfr]

Implements #828
Implements #391

pn.pane.Markdown("""
    # Markdown syntax

    # Title h1

    ## Title h2

    ### Title h3

    #### Title h4

    ## Paragraphs

    Lorem ipsum dolor sit amet, vis suscipit expetendis ut, laoreet instructior at vel, nullam efficiantur sit ex. Ei graeco quaeque omittam vis, qui no bonorum gloriatur. Ut sed aliquam convenire, enim nihil efficiendi eu sea. Ne affert eripuit minimum vis, te vel docendi interesset, vix ea quot albucius gloriatur.

    Lorem ipsum dolor sit amet, vis suscipit expetendis ut, laoreet instructior at vel, nullam efficiantur sit ex. Ei graeco quaeque omittam vis, qui no bonorum gloriatur. Ut sed aliquam convenire, enim nihil efficiendi eu sea. Ne affert eripuit minimum vis, te vel docendi interesset, vix ea quot albucius gloriatur.

    ## Quoted text

    In the words of Abraham Lincoln:

    > Pardon my French

    ## Syntax highlighting

    Some basic Git commands are:

    ```text
    git status
    git add
    git commit
    ```

    ```python
    def some_function():
        return 1
    ```

    ```javascript
    function some_function () {
        return 1
    }
    ```

    ## Bullet Points

    - George Washington
    - John Adams
    - Thomas Jefferson

    ## Ordered List

    1. James Madison
    2. James Monroe
    3. John Quincy Adams

    ## Nested list

    1. First list item
        - First nested list item
            - Second nested list item
""", sizing_mode='stretch_width').show()

[codecov]

Codecov Report

Merging #881 into master will decrease coverage by <.01%.
The diff coverage is 74.35%.

@@            Coverage Diff             @@
##           master     #881      +/-   ##
==========================================
- Coverage   69.71%   69.71%   -0.01%     
==========================================
  Files         104      104              
  Lines       10554    10587      +33     
==========================================
+ Hits         7358     7381      +23     
- Misses       3196     3206      +10

Impacted Files	Coverage Δ
panel/tests/pane/test_markup.py	100% <100%> (ø)	⬆️
panel/tests/pane/test_base.py	84% <50%> (-16%)	⬇️
panel/pane/markup.py	51% <55.55%> (-0.75%)	⬇️
panel/util.py	66.39% <75%> (+0.3%)	⬆️

---

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