feat: support linting stdin

[llllvvuu]

Similarly for fixit fix. The API is inspired by Prettier.

Summary

Related to #122 but does not fully resolve it. It supports a lint-as-you-type workflow, whereas only a lint-on-save workflow could be supported before. Full resolution of #122 would require #387.

Test Plan

Extended smoke tests.

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[llllvvuu]

CLA submitted

[llllvvuu]

Just took a look at ufmt CLI and happy to make the changes. So to confirm, intended usage is

fixit lint - path/to/foo.py  # formats stdin, treating it as if it is located at path/to/foo.py

[amyreese]

Just took a look at ufmt CLI and happy to make the changes. So to confirm, intended usage is

fixit lint - path/to/foo.py  # formats stdin, treating it as if it is located at path/to/foo.py

Correct :)

[llllvvuu]

Just made the changes, thanks again for the ufmt reference!

I wasn't able to leave cli.py 100% unchanged since fixit fix needs to disable --interactive (as STDIN is occupied) and redirect Results to STDERR (as STDOUT is occupied). But let me know what you think

[amyreese]

Thank you!