feat: remove plenary async

[neo451]

• remove the AsyncExecutor Class that depends on plenary.nvim
• use simple callback style for async operation.
• properly test and simplify all find operations
  • find_tags
  • find_backlinks
  • find_links (todo)
  • find_notes

PR Checklist

• The PR contains a description of the changes
• I read the [CONTRIBUTING.md] file
• The CHANGELOG.md is updated
• The changes are documented in the README.md file
• The code complies with make chores (for style, lint, types, and tests)