refactor(installer): skip unstable headless update

[kylo252]

Description

• refactor(installer): skip unstable headless update
• fix(installer): verify that packer is installed
• refactor(packer): skip dodgy snapshot feature

fixes #3358, #3311

How Has This Been Tested?

see #3338 (comment)

[LostNeophyte]

some problems with the backups:

without --overwrite

• any run: empty ignore file is added to LUNARVIM_CONFIG_DIR.old (but it happened before this pr as well, not sure it it's needed)
• 1st run: (may be related to the next one) empty LUNARVIM_RUNTIME_DIR.old is created (when no LUNARVIM_RUNTIME_DIR was present before)
• 2nd run: LUNARVIM_RUNTIME_DIR is backed up to LUNARVIM_RUNTIME_DIR.old/data(name 'data' probably comes from the last folder in LUNARVIM_RUNTIME_DIR) and not LUNARVIM_RUNTIME_DIR
• 3rd run:
  • overrides LUNARVIM_CONFIG_DIR
  • says it backed up LUNARVIM_BASE_DIR but it just deleted it leaving LUNARVIM_BASE_DIR.old untouched
  • gives an error and stops at LUNARVIM_RUNTIME_DIR (I think it should check if .old files exist before doing any backups and error out if they exist)

with --overwrite

• 1st run: (same as before) empty LUNARVIM_RUNTIME_DIR.old is created (when no LUNARVIM_RUNTIME_DIR was present before)
• 2nd run:
  • LUNARVIM_BASE_DIR, LUNARVIM_CONFIG_DIR are backed up and LUNARVIM_RUNTIME_DIR.old has an empty 'data' sub directory

[LostNeophyte]

Now the only problem i can see now is if you have the backups already and you run the installer without overwrite, it will still run mv. I personally think it should abort the install in that case and tell the user to use overwrite if they don't need backups or someting like that.