Enhancement: Support pull and push on submodules

[estruyf]

Support pull and push submodules. If enabled, during the git sync action, the latest changes from the submodule should be pulled and pushed.

[estruyf]

New submodule settings:

{
  "frontMatter.git.submodule.push": true,
  "frontMatter.git.submodule.pull": true,
  "frontMatter.git.submodule.branch": "main",
  "frontMatter.git.submodule.folder": ".frontmatter"
}