Markdown: multiple improvements and fixes

Summary:
## Improve & fix detection of bold/italic text

BUG: 390309

In bold and italic text highlighting, underscores and asterics aren't allowed inside, for example `**some_text**` isn't highlighted correctly. I have improved regex to detect bold and italic text: now asterisks, underscores and escapes are allowed inside.
{F7273185}

## Improve fenced code blocks

The way to write fenced code blocks varies depending on the Markdown/MultiMarkdown implementation. Some support only 3 backticks, others more than 3 backticks, others both; and others 3 tildes (`~`). [1]

Therefore, I think it's a good idea to support all implementations: code blocks can be written with 3 backticks or more, or with 3 tildes or more . Other editors, such as Visutal Studio Code, Atom and Sublime Text highlight fenced code blocks in this way.
Unfortunately, this is only possible using dynamic rules with several RegExpr, which isn't very optimal. I have no problem modifying that if you don't like it.
{F7273187}

**Also:**
* Add folding in fenced code blocks.
* Add more keywords to identify languages.
* Add to highlight in fenced code blocks the languages that are already loaded by default, since they are included via IncludeRules (JavaScript, TypeScript, JSX, SQL, Mustache/Handlebars, reST & Doxygen).
* Add more languages in the code blocks. Some of the most popular languages are included [3] [4]: C, Go, Java, JavaScript, TypeScript, Matlab, Perl, R & Ruby.

## Improve code blocks

* Highlight single code with more than one backticks. [2]
   {F7273189}
* Fix: highlight indented code only after an empty line.
   {F7273193}

## Improve highlighting of links and references
{F7273195}

## Improve metadata highlighting

Previously, some metadata Keys were highlighted anywhere in the document.  Now the metadata is highlighted only on the first line of the Markdown document. [6] [7]
{F7273196}

## Improves list detection

Through dynamic rules, the indentation of lists is captured, in order to correctly highlight the indented content in them.
Now the text within the lists is highlighted using "dsNormal", so as not to saturate Markdown documents with many colors.
{F7273197}

## Add support of inline HTML

`IncludeRules` is used to highlight only HTML tags. [8]

## Others

* Highlight checkboxs in lists.
   {F7273202}
* Add escape characters [5].
* Add `##Alerts` and `##Modelines` in the comments.
* Some minor improvements, such as replacing some RegExpr rules and adding `column="0"` in some rules.

**Sources:**
* [1] Fenced Markdown code blocks: <https://meta.stackexchange.com/questions/125148/implement-style-fenced-markdown-code-blocks/143705#143705>
* [2] Markdown Syntax Documentation. Code: <https://daringfireball.net/projects/markdown/syntax#code>
* [3] GitHut - Programming Languages and GitHub: <https://githut.info/>
* [4] Most Popular and Influential Programming Languages of 2018: <https://stackify.com/popular-programming-languages-2018/>
* [5] Markdown Syntax Documentation. Backslash escapes: <https://daringfireball.net/projects/markdown/syntax#backslash>
* [6] MutiMarkdown Metadata: <https://fletcher.github.io/MultiMarkdown-5/metadata.html>
* [7] Markdown metadata format: <https://stackoverflow.com/questions/44215896/markdown-metadata-format>
* [8] Markdown Syntax Documentation. Inline HTML: <https://daringfireball.net/projects/markdown/syntax#html>

Reviewers: cullmann, dhaumann, #framework_syntax_highlighting

Reviewed By: dhaumann, #framework_syntax_highlighting

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D23371