fix Symbol.prototype.description

[romainmenke]

this is the big one

I am not very confident about this code and this change might introduce new bugs :/
What I wanted to do :

• only ship the entire Symbol polyfill to browsers that really need it. Not to every browser that needs Symbol.prototype.description
• still have a working polyfill for Symbol.prototype.description in all supported browsers

The only way I could think of to do that, was by using a Proxy in modern browsers.
This does introduces duplicate logic and duplicate tests.

I am definitely not an expert on Symbol and might have overlooked nuances when mangling the code for both polyfills.

ci is still running for this one : reference branch

[romainmenke]

@JakeChampion I forgot to finalise the browsers list in config.toml here.
When double checking everything I noticed that Safari 9 doesn't have Proxy.

So I tweaked the polyfill to have a minimal implementation for browsers without Proxy.