Don't pollute completions with globals in templates with syntax errors

[dfreeman]

We fixed this for many cases in #442, but had an outstanding issue with embedded templates.

The root of the problem was essentially that I didn't want to plumb a MappingTree through a few layers of function indirection, and so we were answering the "is this completion being requested within a template?" question in two passes. Unfortunately, in cases with template a syntax error where the template was in the same file as the backing class, the second pass lookup of the mapping tree would fail, because the transformed offset would fall directly on the border between the template and the real JS/TS code.

This change fixes my original sin and plumbs through the relevant MappingTree as part of the transformed offset lookup process.

Fixes #566