initial illumos support

[hadfl]

Like in PR #79 I hit this issue wile trying to build tree-sitter on OmniOS (an illumos distribution).

This is a headless development system, so no webbrowser available.

running 5 tests
test test_open_chrome ... ignored
test test_open_firefox ... ignored
test test_open_safari ... ignored
test test_open_webpositive ... ignored
test os::tests_xdg::test_xdg_open_local_file ... ok

test result: ok. 1 passed; 0 failed; 4 ignored; 0 measured; 0 filtered out; finished in 1.01s

     Running tests/common.rs (target/debug/deps/common-85fea5dcb22e6b1b)

[amodm]

Thanks for your contribution. I'm merging this despite some of the test failures as they seem more to do with the lints in +beta. I'll be fixing those before the release, and update here when the release is done.

[amodm]

@hadfl this has now been released as v0.8.14