feat!: default to config at .config/lintrunner.toml

[aljazerzen]

Adds .config/lintrunner.toml and .config/lintrunner.private.toml to the end of default config paths.

Idea here is that having too much dot files in the repo root is a mess. Alternative is a .config/, which alieviates the problem a lot. See https://dot-config.github.io/ and https://github.com/natemoo-re/proload/issues/34.

This is potentially a breaking change, since project that currenty default to .lintrunner.toml but also has .config/lintrunner.toml will now have root config overridden with the .config.

The chance of this happening are very low, so I don't think this will cause any problems.

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Alternative is to put the new default in the front of the list, but then we would have to change the full default to:

.config/lintrunner.toml, .config/lintrunner.private.toml, ../.lintrunner.toml, ../.lintrunner.private.toml

... since this the last two paths must be relative to .config, which is confusing.