Add language support for Mermaid diagrams

[Alhadis]

Description

This PR adds formal support for Mermaid diagrams, which are already recognised by GitHub's markup processor:

<pre lang="mermaid"><code>flowchart LR
	A[Hard edge] -->|Link text| B(Round edge)
	B --> C{Decision}
	C -->|One| D[Result one]
	C -->|Two| E[Result two]</code></pre>

flowchart LR
	A[Hard edge] -->|Link text| B(Round edge)
	B --> C{Decision}
	C -->|One| D[Result one]
	C -->|Two| E[Result two]

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I've included mermaid example as an alias so that snippets of Mermaid source can be embeddedand highlighted without being physically rendered. It's already in-use by Mermaid's docs.

@lildude I'm gauchely assuming that both the .mmd and .mermaid extensions satisfy our popularity requirements. Correct me if I'm wrong. 😉

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories
    Probably. See the @mention above.
    • Search results for each extension:
      • .mmd: ~10,572 results
      • .mermaid ~810 results
  • I have included a real-world usage sample for all extensions added in this PR
    Each sample was either taken from Mermaid's documentation (which is MIT-licensed), or written by yours truly during development of Alhadis/language-mermaid.
  • I have included a syntax highlighting grammar
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

Yup, usage looks good with a good distribution of the 810 .mermaid files.