LibCore: Add File method to determine absolute path

This will generate absolute paths lexically rather than through a call
to realpath. The motivation for this is to generate absolute paths for
non-existent files in unveil calls, as realpath will not work if the
file does not exist.

Userland/Libraries/LibCore/File.cpp

@@ -207,6 +207,20 @@ String File::current_working_directory()
     return cwd_as_string;
 }
 
+String File::absolute_path(String const& path)
+{
+    if (File::exists(path))
+        return File::real_path_for(path);
+
+    if (path.starts_with("/"sv))
+        return LexicalPath::canonicalized_path(path);
+
+    auto working_directory = File::current_working_directory();
+    auto full_path = LexicalPath::join(working_directory, path);
+
+    return LexicalPath::canonicalized_path(full_path.string());
+}
+
 #ifdef __serenity__
 
 String File::read_link(String const& link_path)

Userland/Libraries/LibCore/File.h

@@ -36,6 +36,7 @@ public:
     static bool exists(const String& filename);
     static bool ensure_parent_directories(const String& path);
     static String current_working_directory();
+    static String absolute_path(String const& path);
 
     enum class RecursionMode {
         Allowed,