Change bar chart numbers and colours to be accessible

[injms]

What

1. Add an array to set the colour of numbers on govspeak bar charts with multicoloured bars based on the colour of the bar itself eg: the yellow bar has black text, the red bar has white text etc.
2. Add an example of a bar chart with multicoloured bars to the component guide for govspeak.

Why

When a number is positioned inside a bar of a barchart, the colour contrast between the background colour of the bar and the default colour of the text (white) isn't high enough in some cases. It should be at least 4.5:1 or above to meet the WCAG 2.1 AA standard and, where possible, 7:1 to meet WCAG 2.1 AAA.

Visual Changes

Before:

After:

[injms]

Because I opened this pull request, I can't approve this. But I do approve of this.

[danacotoran]

👏 does what it's supposed to!