Support <c-t> as navigateBack

[kflu]

C-t is used in vim as jumping back in tag stack. With LSP, this is
normally used for "navigating back". So adding this to vscode neovim for
the same convention.

However, as I tested, it does NOT currently work - it can send
to neovim alright. But I got "E73: tag stack empty", as if c-t were
still bound to neovim's default c-t behavior (jump back in tag stack).
I expected it to have been mapped to workbench.action.navigateBack.
Any hint?

TEST:

npm install vsce
export PATH=`realpath node_modules/.bin`:$PATH
vsce package

code --install-extension vscode-neovim-0.0.83.vsix

[theol0403]

1. you just mapped xnoremap, which is visual mode, there is actually two lines that you must map
2. the vim function you wrote is not necessary and was just made so that C-] works in help files
3. it would be great if you could just do this in your PR: https://github.com/kflu/vscode-neovim/blob/55d64509ac962623e218053ce281c85febf27bb9/vim/vscode-jumplist.vim#L1

Thanks!