fix: Run AstroUpdate before Lazy sync

[slowsage]

AstroNvim switched to using Lazy to manage plugins some weeks back.
For users of AstroNvim:

• Without this PR, topgrade runs Lazy sync (since :Lazy exists via AstroNvim) and quits without updating AstroNvim.
• With this PR, AstroUpdate is called if it exists before checking for existence of Lazy.nvim. AstroUpdate will update the plugins (using Lazy currently) as needed. So the explicit call to Lazy sync in upgrade.vim becomes unnecessary.

I suggest that users of AstroNvim add

  updater = {
    skip_prompts = true,
  },

to their init.lua to automatically confirm and quit AstroNvim after AstroUpdate and skip the redundant call to Lazy sync.
Users not using AstroNvim should be unaffected.

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[SteveLauC]

Except for that doc nitpick, this PR LGTM:)

[slowsage]

Please hold off on merging. Just realized I need to test one more case. Might need to change the comment/code. I'll comment back when its ready. Apologies.

[slowsage]

Turns out the auto_quit option only gets triggered when there are updates. So we need to call quitall to cover that case. It's ready to be merged. Thank you.