Register `.dart_tool` assets as `externalSymbols`

[matthewnitschke-wk]

Dart has a fairly common concept of generated files that can be placed directly in line with source, or referenced within a cache directory within .dart_tool

the .dart_tool directory is gitignored so assets themselves are not locatable without running a build and inspecting deeply nested files within .dart_tool

over_react is an example of a repo which generates these files and relies on them for backing prop classes

These references to .dart_tool symbols are triggering scip analysis errors due to the occurrence existing, but no declaration (since we dont index .dart_tool)

I have a long term vision of supporting a plugin system where libraries like OverReact would be able to adjust the indexing of the scip-indexer (goto definition on a generated prop class could take you to the mixin FooProps {...} instead of the backing dart map in the generated asset), but that vision is a long way off

In the meantime, just register all .dart_tool symbols as an externalSymbol so they are still included/indexed, but the analysis doesn't error out when attempting to index these