Add `--allow-file-access-from-files` to default runtime arguments ... in trusted workspaces

Reasoning: This is an extension for developers which will open local files - of course these should be able to include local files...
See https://stackoverflow.com/questions/4558160/xsl-not-working-in-google-chrome for some details (and the reason that this is not the default in chrome, as it [also isn't in firefox](https://support.mozilla.org/de/questions/1264318)).

As vscode introduced the trusted workspace concept it seems very reasonable to always add this option in trusted workspaces, while leaving it out otherwise. This is also the core difference to #77 (which would be "only if the user specified, and then always).

The issue names no files or tests. Start by locating where default runtime arguments are assembled and where trusted-workspace state is checked; done means the flag is added for trusted workspaces and omitted otherwise, without changing the untrusted behavior described here.

Written by the indexing model from the issue text.