feat: Add Deno lsp support

autoload/ale/handlers/deno.vim

@@ -3,7 +3,50 @@
 
 call ale#Set('deno_executable', 'deno')
 call ale#Set('deno_unstable', 0)
+call ale#Set('deno_lsp_project_root', '')
 
 function! ale#handlers#deno#GetExecutable(buffer) abort
     return ale#Var(a:buffer, 'deno_executable')
 endfunction
+
+" Find project root for Deno's language server.
+"
+" Deno projects do not require a project or configuration file at the project root.
+" This means the root directory has to be guessed,
+" unless it is explicitly specified by the user.
+"
+" The project root is determined by ...
+" 1. using a user-specified value from deno_lsp_project_root
+" 2. looking for common top-level files/dirs
+" 3. using the buffer's directory
+function! ale#handlers#deno#GetProjectRoot(buffer) abort
+    let l:project_root = ale#Var(a:buffer, 'deno_lsp_project_root')
+
+    if !empty(l:project_root)
+        return l:project_root
+    endif
+
+    let l:possible_project_roots = [
+    \   'tsconfig.json',
+    \   '.git',
+    \   bufname(a:buffer),
+    \]
+
+    for l:possible_root in l:possible_project_roots
+        let l:project_root = ale#path#FindNearestFile(a:buffer, l:possible_root)
+
+        if empty(l:project_root)
+            let l:project_root = ale#path#FindNearestDirectory(a:buffer, l:possible_root)
+        endif
+
+        if !empty(l:project_root)
+            " dir:p expands to /full/path/to/dir/ whereas
+            " file:p expands to /full/path/to/file (no trailing slash)
+            " Appending '/' ensures that :h:h removes the path's last segment
+            " regardless of whether it is a directory or not.
+            return fnamemodify(l:project_root . '/', ':p:h:h')
+        endif
+    endfor
+
+    return ''
+endfunction