Add support for Esri Shapefile

[tmcw]

If you're adding support for a new file type, please follow the below steps:

• One PR per file type.
• 👍 Add a fixture file named fixture.<extension> to the fixture directory.
• 👍 Add the file extension to the extensions array in supported.js.
• 👍 Add the file's MIME type to the types array in supported.js.
• 👍 Add the file type detection logic to the index.js file.
• 👍 Add the file extension to the FileType type in index.d.ts.
• 👍 Add the file's MIME type to the MimeType type in index.d.ts.
• 👍 Add the file extension to the Supported file types section in the readme, in the format - [`<extension>`](URL) - Format name, for example, - [`png`](https://en.wikipedia.org/wiki/Portable_Network_Graphics) - Portable Network Graphics
• 👍 Add the file extension to the keywords array in the package.json file.
• 👍 Run $ npm test to ensure the tests pass.
• 👍 Open a pull request with a title like Add support for Format, for example, Add support for PNG.
• 👍 The pull request description should include a link to the official page of the file format or some other source. Also include a link to where you found the file type detection / magic bytes and the MIME type.

---

• Here's the authoritative specification for the format, which includes the magic number / unused details: https://www.esri.com/library/whitepapers/pdfs/shapefile.pdf
• And the implementation from the epic historical file utility https://github.com/file/file/blob/31a82018c2ed153b84ea5e115fa855de24eb46d1/magic/Magdir/esri
• Fixes Add Esri Shapefile support? #247

[bencmbrook]

Looks good to me!