Add support for ASCII STL

[dalgleish]

Add support for ASCII STL files, as requested in #401

Detects the first 6 bytes of the line solid name (where name is optional).

The fixture file is a 2d square.

Binary STL files are more common, but they do no contain any detectable header pattern.

---

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 core.d.ts.
• Add the file's MIME type to the MimeType type in core.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.

[Borewit]

LGTM