Add a displayName to the PageContext

[hailwood]

Presently in the react devtools we have this:

Context.Provider there being the pageContext we create as that is the default name,
but it's possible to name the contexts which makes navigating the react tree much nicer - After this pull request it will display as:

[reinink]

Interesting, we do something similar in Vue to make the devtools nicer to use.

Ping @sebastiandedeyne for review. 👍

[sebastiandedeyne]

Perfect!