fix(engine-core): add shim for old stylesheetTokens internal API

[nolanlawson]

Details

Adds a shim for the old stylesheetTokens internal API on top of the new stylesheetToken API. Neither one should be used by component authors (since they are internal and undocumented), but they are used in the wild, so this unbreaks anyone who is using the old API.

Related:

• feat: light DOM scoped styles #2332
• perf: only add scope token when stylesheets are present #2670

Does this pull request introduce a breaking change?

• ✅ No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• ⚠️ Yes, it does include an observable change.

If you were setting the stylesheetTokens on a template by copying it from one template to the other, it broke due to #2332 and #2670. With this PR, it works like before.

GUS work item

W-11093934

[ravijayaramappa]

Suggestion: Assert on the actual contents of the properties rather than the left hand side and right hand side of the assignment in L32

Suggested change

	expect(withoutStylesheet.stylesheetTokens).toEqual(withStylesheet.stylesheetTokens);
	expect(withoutStylesheet.stylesheetToken).toEqual(withStylesheet.stylesheetToken);
	expect(withoutStylesheet.stylesheetTokens).toEqual({
	hostAttribute: 'x-inspectStylesheets_withStylesheet-host',
	shadowAttribute: 'x-inspectStylesheets_withStylesheet',
	});
	expect(withoutStylesheet.stylesheetToken).toEqual('x-inspectStylesheets_withStylesheet');

[nolanlawson]

I'd rather not tie the test to implementation details of what the generated token looks like. Jasmine toEqual already does deep equality comparison.