Add (Basic) getter methods for the v8 value struct

[rogchap]

This will enable getting of richer data types from the v8 value struct than just currently string

All the basic ToXXX methods have been implemented as defined here:
https://v8.github.io/api/head/classv8_1_1Value.html#a838af7d16f0ce87b4411beb178383db4

However ToBigInt() and ToObject() have NOT yet been implemented. The reason is that Object represents everything that is not a primitive type: eg. Functions, Objects, Promises, Dates, Regex etc. Implementing all of the Object type would make this PR too large, and take quite a bit of time to implement.

Was hoping to include this PR in the next release, which already has a number of features people are waiting for to be released.

As such, I think it's best to un-export these methods (to not break the public API), get this PR merged, do a release and continue work on Object (and BigInt) in later PRs.

[tmc]

This looks great! We should consider issues for the bigInt and object implementations. I like the fmt.Formatter support addition.

[rogchap]

@tmc Thanks. I've added those two issues as suggested. ☝️