fix: treat log files as plaintext

[SSDWGG]

Summary

• map .log files to plaintext highlighting
• add a regression test for the extension mapping

Fixes #5253

Tests

• npx vitest run src/tests/helpers/mappings.spec.ts

[mejo-]

@SSDWGG thanks as well for this contribution. As far as I can see, Text doesn't open .log files at all per default. So I wonder whether it's useful to add syntax highlighting conditions for it. Can you explain your usecase a bit?

Regarding the unit test, I think it can be dropped in any case. It doesn't test much except that the object key you introduced to mappings.ts has the value your put there.