feat: add a mechanism to retrieve all context (with defaults)

[pwrmiller]

Previously, all users of constructs would have to know beforehand the keys in the context to expect, and use either tryGetContext or getContext to retrieve a specific key from context.

This provides a mechanism to get the full context of a node in the construct tree, with overridable defaults.

The getAllContext method allows users to retrieve all context. The interface for getContext and tryGetContext remains unchanged.

I have also fixed a small typo in the library.

[mrgrain]

Apologies for the delay @pwrmiller