Add polyfill tag

[theofidry]

This allows this package to be found when looking at the polyfill(s) tagged Composer packages

[narfbg]

I'd also add "compat" to that.

[theofidry]

Is it used though? Most of the polyfills in the wild are from Symfony and none of them are tagged as "compat" :/

[narfbg]

If I was looking for such a package, I'd search for "compat" instead of "polyfill" - that's what I'm saying.

[svenluijten]

@narfbg this package is already the first result when searching for "compat" on https://packagist.org, what would an extra keyword do?

[glensc]

but if you search "polyfill", it's third

as you see the description matches "polyfill" so i think it's fair to include it in keywords too.

[paragonie-scott]

Keywords cost us nothing, so, I'm okay with including both.