[XY Charts] Axis and grids style update

[gvnmagni]

A few improvements that we can make on grids and axis without causing any disruption:

Monospaced font

The introduction of monospaced font is currently under testing and can be foloowed here: elastic/kibana#233725

Labels colors

In order to be consistent with EUI and get a stronger contrast with background, we should use colors.textParagraph from EUI tokens

No open grids

I would avoid having "open" grids and I would always display a border at the edge of the chart area even if it doesn't match with a tick gridline, this would help us visually closing the chart area making it look more compact and clean.

Vertical axis first and last tick label

Currently, all ticks labels are vertically aligned to their respective tick grid line, it would awesome if these labels wouldn't go over the chart area described in the previous point. In order to do this, the first and last label should be forced to be "inside" of that area. (look at label 0 [zero] within the screenshot for reference)

[gvnmagni]

Let's check for grids colors as well, they should use colors.borderBaseSubdued from EUI (or a different token that reflects that token, not entirely sure how we connect to these color variables)