Fixes #2797
What this PR solves / how to test:

Previously, Pages Functions exposed context.data like this:

const data = {};

const context = {
	...,
	data
};

Which meant that users could set context.data.foo = false and it would work fine, since it's just a reference to the top-level data per request, but the second they tried to override context.data entirely, such as context.data = {foo: false}, the reference to the original data is lost, so this data wouldn't actually show up.

This PR fixes that by adding a getter/setter pair to data, so that folks can override this without any issues.

Author has included the following, where applicable:

• Tests
• Changeset (Changeset guidelines)

Reviewer is to perform the following, as applicable:

• Checked for inclusion of relevant tests
• Checked for inclusion of a relevant changeset
• Checked for creation of associated docs updates
• Manually pulled down the changes and spot-tested