fix: mac support

[LindirQuenya]

This PR fixes four issues that affected MacOS builds.

1. state.mainFolderPath (and hence configFolder, too) was being set to DIR/Flashpoint.app/Contents/MacOS, rather than DIR. ([BUG] MacOS: Launcher cannot load its own config and preferences #296)
2. The launcher.log was being placed in DIR/Flashpoint.app/Contents/MacOS, rather than DIR.
3. state.config.flashpointPath was interpreted as relative to the current working directory, which is set to / by Finder. This was fixed by interpreting a relative flashpointPath as relative to the configFolder, not relative to the current working directory.
4. The path for the lang jsons was defaulting to DIR/Flashpoint.app/Contents/MacOS/lang, instead of DIR/Flashpoint.app/Contents/lang.

All of the fixes only take effect when on Mac, so they shouldn't break any other platforms.

[coveralls]

Pull Request Test Coverage Report for Build 1800548919

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 60.163%

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Totals
Change from base Build 1766549299:	-0.1%
Covered Lines:	360
Relevant Lines:	548

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💛 - Coveralls