This simply changes the flow and API for getting IPFS proxy instance,
does not implement UX nor decrease the size of injected content script.

This change adds async `enable` function that takes optional list of commands
to grant access to via user prompt.

The goal is to move away from synchronous way of accessing API instance
and provide UX incentive to use `window.ipfs.enable` instead.

When called without any arguments, command will just return API instance
equal to the old `window.ipfs` or throw an error if IPFS Proxy is
disabled in Preferences.

When called  with options object `{ commands: ['id','peers'] }`
access rights for specified commands will be validated:

- if any of the commands is denied or blocked, function will throw
- if any of the commands require user approval, user will be presented
with a single prompt dialog that lists all requested permissions and URL
that requests them
  - if user approves, ACLs are saved and future calls will not trigger
  prompt
  - if user denies, ACLs are saved and an error is thrown for current
  and all future executions (unless user removed scope from blacklist)

TODO (to be addressed in future commits)

- add deprecation warning to API calls executed on `window.ipfs`
- improve UX of permission dialog
- add ability to return `ipfsx` version fo the API
- disable `window.ipfs` injection via manifest in Chromium
- stop exposing methods on `window.ipfs`
    - minimize the size of content script responsible for `window.ipfs`
    - lazy-init IPFS Proxy client on first call to `window.ipfs.enable()`

- also switch to terser-webpack-plugin

acl and api looked too similar and made code hard to follow,
ipfs api core uses 'command' term to identify each endpoint
so we follow that practice here

To avoid breaking changes we should deprecate old error codes first.

This commit:
- restores `permission` attribute and adds a deprecation
warning when it is accessed.
- adds error `code` to simplify error handling and alight convention
with js-ipfs
- updates docs

This adds an opt-in ipfsx experiment.
In short, if `experiments:{ipfsx:true}` is passed `window.ipfs.enable`
will return IPFS API instance wrapped in ipfsx prototype from
https://github.com/ipfs-shipyard/ipfsx

```
let ipfs = await window.ipfs.enable({
    commands: ['add','files.addPullStream'],
    experiments: { ipfsx: true }
})
```

This change means no command can be run without explicit approval from
the user.

Rationale can be found in
#619 (comment)