`{extensions}`: Improve `Extension` interface methods naming.

#### Actual

Currently the extensions component defined in:

https://github.com/graphql-go/graphql/blob/58689e0742f217137ac0c82b16e2d658e0cc1853/extensions.go#L32-L33

Contains methods that are named in a way that could be clearer, for example:

https://github.com/graphql-go/graphql/blob/58689e0742f217137ac0c82b16e2d658e0cc1853/extensions.go#L40-L41

^ It is not clear if the hook executes before or after the `Parse` process started.

#### Expected

We want to update the `Extensions` interface methods to a clearer naming using precise prefix, eg:

`ParseDidStart` -> `BeforeParseHook`

Using the same re-naming strategy: `Before/AfterHook` we want to update the other methods.

For backwards compatibility let's keep the existing methods as a fallback.

#### Notes

- This naming improvement was found while working in a University Thesis via the following PR: https://github.com/chris-ramon/thesis-graphql-go/pull/66