Support Haskell2010 as pragma

`Haskell2010` is an official GHC extension (https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/table.html) and used in [`yi-editor/yi`](https://github.com/yi-editor/yi) (https://github.com/yi-editor/yi/blob/13fa0f7c598a115485572b66fed5bc3cf383d0ef/yi-core/src/Yi/PersistentState.hs#L5), for example. I suggest to support this extension by replacing

```haskell
readExtension :: String -> Maybe Extension
```
with

```haskell
readExtension :: String -> [Extension]
```

and yield `default2010Extensions` in the case of `Haskell2010`.

If this is fine, I will create a pull request.

Start at the readExtension entry point and compare how existing extension names are converted to Extension values. Trace its callers to account for the proposed return-type change, then verify that Haskell2010 yields default2010Extensions and that Haskell2010 pragmas are accepted as intended.

Written by the indexing model from the issue text.