Make the cli's csharp feature actually turn csharp off

[Lstarsky0]

wit-bindgen-csharp is optional = true in the cli, but the cli also depends on
wit-bindgen-test unconditionally, and that crate depended on wit-bindgen-csharp
outright. So the feature never actually removed anything:

$ cargo tree --no-default-features -i wit-bindgen-csharp
wit-bindgen-csharp v0.60.0 (crates/csharp)
├── wit-bindgen-cli v0.60.0 (.)
└── wit-bindgen-test v0.60.0 (crates/test)

After this it's out of the graph for --no-default-features and for any single
non-csharp language, and still in for the default build and for
--no-default-features --features csharp.

crates/test is the only test driver that links a bindgen crate — the others shell
out — and it does so for CSProject, to write the .csproj. So mod csharp and the
Language::Csharp variant go behind the same feature. With csharp off the harness
no longer knows the cs extension and falls through to the --custom lookup, which
says the extension is unknown. Happy to keep the variant always present and error at
run time instead if you'd rather.

Closes #1510

[alexcrichton]

Thanks! I'm a bit hesitant to do this though because it's a pretty minor gain (removing a crate at compile time), and this otherwise makes the internal implementation of the test framework inconsistent where C# gets a cargo feature but nothing else does.

One thing that I'm not certain of, however, is the original motivations of #1510. Do you have your own motivations for fixing this issue? Or are you mostly looking to contribute and that seemed a good bug to fix?

[Lstarsky0]

No motivation of my own here — I'm not a C# user and I don't build with --no-default-features. I was looking for issues I could verify end to end without a language toolchain, and this one fit.

I'd also narrow what I wrote in the description: the feature does still gate the csharp subcommand, it's only the build graph that's unaffected. So the gain is smaller than I made it sound, and I agree it isn't worth being the one place in the test framework with a cargo feature.

Closing. Leaving #1510 open in case someone shows up who actually wants the smaller build.