Revert Startup Behaviour Changes

[alex-courtis]

Several changes to startup behaviour have recently been made:

• 0417d91
• fix(#1270): open_on_setup_file does not override open_on_setup, hijack_directories does not override startup behaviour #1618
• fix(#1629): nvim start with file named *NvimTree* opens tree instead of buffer #1634
• fix(#1664): respect hijack_directories.enable on startup when not open_on_setup #1665

These have resulted in regressions:

• hijack_directories Not Respected When Not open_on_setup #1664
• Bug: nvim-tree does not respects the call functions when opening nvim nvim . #1602
• Looks like `nvim .` doesn't open the tree anymore? #1660
• nvim-tree opening when editing non existant file with open_on_setup_file = false #1663

Revert all the above changes. Add a new mechanism to handle startup behaviour: #1669