Bugfix in module executable.lua on Windows (#450)

Now using 'vim.fn.exepath' to retrieve the correct value for the flutter
executable, when using fvm on windows. Previously the plugin set the
flutter executable to the UNIX script ("flutter") but Windows has its'
own version ("flutter.bat").

Author: obemu

lua/flutter-tools/executable.lua

@@ -103,6 +103,7 @@ local function _flutter_bin_from_fvm()
   local fvm_root =
     fs.dirname(fs.find(".fvm", { path = luv.cwd(), upward = true, type = "directory" })[1])
   local flutter_bin_symlink = path.join(fvm_root, ".fvm", "flutter_sdk", "bin", "flutter")
+  flutter_bin_symlink = fn.exepath(flutter_bin_symlink)
   local flutter_bin = luv.fs_realpath(flutter_bin_symlink)
   if path.exists(flutter_bin_symlink) and path.exists(flutter_bin) then return flutter_bin end
 end