Add syntax highlighting to CSV and TSV files

[Alhadis]

Description

This pull-request adds two recently-written grammars to highlight CSV and TSV files. While these are normally rendered as searchable tables on GitHub (and thus their textual contents are rarely displayed), the same isn't true for embedded CSV/TSV code-blocks in Markdown:

```csv
Name,Age,Country
John,38,Australia
Collin,,South Africa
```

This was requested in a GitHub discussion thread, though I don't remember where. It's been buried under too many notifications for me to easily find it.

Checklist

• I am changing the source of adding a syntax highlighting grammar:
  • URL: Alhadis/language-dsv
  • License: ISC

[Alhadis]

/cc @lildude This should be relatively low-hanging fruit as far as pull-requests go.

[DecimalTurn]

This should close #7630