fix: preserve queued keys at picker launch (#2274)

[drmikehenry]

Description

Ensure that any keystrokes that are queued at picker launch are processed only after the picker's mode (insert or normal) has been chosen, preserving their intended meaning.

Previously the picker's mode was set by simulating keystrokes via nvim_feedkeys(simulated_keypresses, "n"). In the absence of queued keystrokes, this works fine; but if the user is able to queue keystrokes before the call to nvim_feedkeys(), those queued keystrokes are processed before the simulated keystrokes that change the picker's mode. Because of this unexpected ordering, the user's queued keystrokes may appear to be ignored or may cause the picker to start in the wrong mode.

For example, consider the below normal-mode mapping:

:nnoremap <space>ff :Telescope find_files<CR>

Upon launching the picker via <space>ff, Neovim is already in normal mode. To switch to insert mode in the picker, Telescope previously used a call to nvim_feedkeys("A", "n"), simulating a keypress of A to enter insert mode at the end of the current line. This A would not be processed until all previously queued user keystrokes have been processed, causing issues.

In real-world use, problems occur when the user types <space>ff followed quickly by characters intended as fuzzy match text. This can be demonstrated using nvim_feedkeys() as shown below.

:call nvim_feedkeys("\<space>ff" . "apple")

The user intended to search for apple, but the a is misinterpreted as a request to enter insert mode at end of line, after which pple is inserted; subsequently, Telescope's simulated A is then appended, resulting in a search string of ppleA.

To ensure that Telescope's simulated keypresses are processed first, an additional i flag is now passed to nvim_feedkeys(), causing the simulated keypresses to be inserted at the start of the typeahead buffer ahead of any user keystrokes.

Fixes #2274.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Tested interactively and via :call nvim_feedkeys() as explained above

Configuration:

• Neovim version (nvim --version):

  NVIM v0.9.1
  Build type: Release
  LuaJIT 2.1.0-beta3
  
     system vimrc file: "$VIM/sysinit.vim"
    fall-back for $VIM: "/__w/neovim/neovim/build/nvim.AppDir/usr/share/nvim"
  
  Run :checkhealth for more info
  

• Operating system and version:

  $ lsb_release -a
  No LSB modules are available.
  Distributor ID: Ubuntu
  Description:    Ubuntu 22.04.2 LTS
  Release:        22.04
  Codename:       jammy
  

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[drmikehenry]

See additional discussion in #2618.