Add composer.json

[quentin-st]

This PR adds a composer.json file, allowing us to require this library as a composer (PHP npm) package.

We'll have to publish this on packagist.org before being able to composer require mdi/svg.

There is literally 0 maintenance once the package is published: you can use GitHub hooks to tell packagist when a new version is published.

@Templarian : I don't know if we can use the mdi/ namespace. A composer package published on packagist already uses it, but I don't know if that means that we cannot use it. We can try - if it does not work, we can still use a package name like material-design/svg.

[Templarian]

There doesn't seem to be much demand for this. I'm leaning toward just closing this PR.

[Milad]

Not a good idea. composer is for PHP, please keep it this way, and use npm for front-end dependencies.

[tip2tail]

With projects sich as the new https://github.com/symfony/asset-mapper there will be more of a demand for this - it would be very helpful for my use-case already.

Can this be considered @Templarian ?