Add SVG data

Nobody has claimed this yet.

It would be helpful to have also SVG related data.

Optimizers like svgo heavily use such data, see https://github.com/svg/svgo/blob/master/plugins/_collections.js .

Some svgo plugins also define extra data:
- https://github.com/svg/svgo/blob/master/plugins/removeUselessStrokeAndFill.js
- https://github.com/svg/svgo/blob/master/plugins/removeUnusedNS.js
- https://github.com/svg/svgo/blob/master/plugins/removeUnknownsAndDefaults.js
- https://github.com/svg/svgo/blob/master/plugins/_path.js
- https://github.com/svg/svgo/blob/master/plugins/_transforms.js
- https://github.com/svg/svgo/blob/master/plugins/convertStyleToAttrs.js
- https://github.com/svg/svgo/blob/master/plugins/cleanupNumericValues.js
- https://github.com/svg/svgo/blob/master/plugins/cleanupListOfValues.js

Further interesting data for SVG: Which attributes allow color-, dimension-, number-, string-, url-values?
This avoids blindly iterating over all values and trying to match them with multiple regexps for example.

Start by reviewing the repository's existing data structures and entry points for related web-platform data. Compare the linked SVGO collection and plugin files, then define the SVG metadata and value categories that fit the project's conventions. Done means the requested SVG data is represented consistently and covers the identified optimizer-related cases.

Written by the indexing model from the issue text.