Proof of concept: Integrate httpgd

[ManuelHentschel]

Update: To make styling easier, I've removed some hardcoded styles in a fork of httpgd.
The viewer now contains a very rudimentary plot history, which highlights a plot on hover but does not do anything else and lets the user select a plot

What problem did you solve?
This PR contains a very basic integration of httpgd to display plots.
A big advantage of the SVG format used here is that it can be styled using the CSS variables provided by vscode.
Between the two screenshots below, only the color theme and font family (in vscode's settings) were changed, the plot updated automatically.
The added typescript code is mostly just copied from here.

Screenshots:

How can I check this pull request?
Install httpgd from this branch, copy the following code to your .Rprofile and call plotTest() in an R terminal created by the extension.

options(vsc.plot = FALSE)
options(device = function(...) {
  httpgd::hgd(
    silent = TRUE
  )
  .vsc$request('httpgd', url=httpgd::hgd_url())
})

plotTest <- function(){
  plot(sin(1:30/10))
  plot(sin(1:90/10))
  plot(sin(1:300/10))
}

cc: @nx10, @renkun-ken

[BerriJ]

I love this!