Add support for Scarb in cairo files (#4669)

* Add support for Scarb in `cairo` files
* specify if linter should run on saved

ale_linters/cairo/scarb.vim

@@ -0,0 +1,31 @@
+" Author: 0xhyoga <0xhyoga@gmx.com>,
+" Description: scarb for cairo files
+
+function! ale_linters#cairo#scarb#GetScarbExecutable(bufnr) abort
+    if ale#path#FindNearestFile(a:bufnr, 'Scarb.toml') isnot# ''
+        return 'scarb'
+    else
+        " if there is no Scarb.toml file, we don't use scarb even if it exists,
+        " so we return '', because executable('') apparently always fails
+        return ''
+    endif
+endfunction
+
+function! ale_linters#cairo#scarb#GetCommand(buffer, version) abort
+    return 'scarb build'
+endfunction
+
+call ale#linter#Define('cairo', {
+\   'name': 'scarb',
+\   'executable': function('ale_linters#cairo#scarb#GetScarbExecutable'),
+\   'command': {buffer -> ale#semver#RunWithVersionCheck(
+\       buffer,
+\       ale_linters#cairo#scarb#GetScarbExecutable(buffer),
+\       '%e --version',
+\       function('ale_linters#cairo#scarb#GetCommand'),
+\   )},
+\   'callback': 'ale#handlers#cairo#HandleCairoErrors',
+\   'output_stream': 'both',
+\   'lint_file': 1,
+\})
+