#1880 coverage configuration 'all' option adds coverage for untested files

[miyasudokoro]

I implemented #1880, which is to enable the configuration "all" to provide coverage measurement and reporting for untested/untouched files.

1. The core code change is in test-runner-coverage-v8 index.ts, based on the c8 implementation for "all". It generates empty reports for all included-not-excluded non-test files in the base coverage directory.
2. To improve performance for large projects, I skip generating empty reports for files present in the local cache of test-runner-coverage-v8 index.ts, because if they are in the local cache, they have already had some coverage measured.
3. The generated empty reports are then passed through the same v8-to-istanbul code as normal reports to ensure uniform reporting. To more easily reuse the code and improve readability, I split the existing v8ToIstanbul function into multiple smaller functions.

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