Allow a key sequence binding like `j,k` to not prevent user from insert `j`

Description of the new feature/enhancement

You can bind a key sequence like j,k to a function or script block by Set-PSReadLineKeyHandler -Key 'j,k' -ViMode Insert -Function ViCommandMode, but a user cannot insert a character j after that.

Proposed technical implementation details (optional)

Quoted from https://github.com/PowerShell/PSReadLine/issues/1701#issuecomment-664020465

In vim, jk or jj to exit insert mode is super common. Usually it works like this:

1. You press <kbd>j</kbd>
2. <kbd>j</kbd> is written to the buffer
3. A timer starts
4. If you press the next key in the sequence before the timer is up, the j is removed from the buffer and the command fires
5. If the key you press next isn’t in the sequence, or the timer is up then it’s treated like a literal

[Drllap]

This is what I added to my profile to make jk go to comand mode:

$j_timer = New-Object System.Diagnostics.Stopwatch

Set-PSReadLineKeyHandler -Key j -ViMode Insert -ScriptBlock {
    [Microsoft.PowerShell.PSConsoleReadLine]::Insert("j")
    $j_timer.Restart()
}

Set-PSReadLineKeyHandler -Key k -ViMode Insert -ScriptBlock {
    if (!$j_timer.IsRunning -or $j_timer.ElapsedMilliseconds -gt 1000) {
        [Microsoft.PowerShell.PSConsoleReadLine]::Insert("k")
    } else {
        [Microsoft.PowerShell.PSConsoleReadLine]::ViCommandMode()
        $line = $null
        $cursor = $null
        [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref]$line, [ref]$cursor)
        [Microsoft.PowerShell.PSConsoleReadLine]::Delete($cursor, 1)
        [Microsoft.PowerShell.PSConsoleReadLine]::SetCursorPosition($cursor-1)
    }
}

[SeeminglyScience]

Yeah, j will be bound after that and won’t be treated as a literal character. I think that’s just an example.

Nah jk or jj (what I use) to exit insert mode is super common. Usually it works like this:

1. You press <kbd>j</kbd>
2. j is written to the buffer
3. A timer starts
4. If you press the next key in the sequence before the timer is up, the j is removed from the buffer and the command fires
5. If the key you press next isn’t in the sequence, or the timer is up then it’s treated like a literal