Improve Diagnostics #42
Description
Diagnostics for an import map should include locations/spans of errors in the source file, to make printing and using the diagnostics in language servers more useful.
@dsherret just added a strict parse mode to https://crates.io/crates/jsonc-parser, which can be used to parse and walk the JSON structure to improve the diagnostics. We should use that for parsing the import map and generating diagnostics.