`<Tab>` and `<BS>` insertMode keybindings

Describe the bug
Expected to be able to change <Tab> and <BS> (backspace) into a different command in insert mode.

To Reproduce
In settings.json:

 "vim.insertModeKeyBindingsNonRecursive": [
        {"before": ["<Tab>", "<Tab>"], "after":["<Esc>"]},
        {"before": ["<Tab>", "<BS>"], "after":["<Tab>"]},
    ]

1. with the above in settings.json
2. edit a file.
3. Go to Insert Mode
4. Expected <Tab><Tab> to go back to Normal Mode
5. also expect <Tab><BS> to insert a <Tab>.

Expected behavior
Expect to be able to remap <Tab> and <BS> in insert mode.

Environment (please complete the following information):

• Extension (VsCodeVim) version: 1.18.5
• VSCode version: 1.51.1
• OS: macOS Mojave 10.14.6

Additional context

I like to do <Tab><Tab> to <Esc>, but this is annoying when I actually want to input a <Tab> so that's why I also have <Tab><BS> to actually input a <Tab>.

I'm using Colemak keyboard layout, so <BS> is the capslock, so not so bad.

Reproduce the insert-mode mappings from settings.json using the reported Tab and Backspace sequences. Trace how VSCodeVim processes insert-mode keybindings, then verify that both sequences trigger their configured actions without breaking ordinary Tab input.