New template:slope chart ✨ + adjust helpers

[henryjameslau]

Key changes to createDirectLabels helper function, in the options object: - labelStrategy can be set as 'firstLast'

• includeDataValue is a boolean and make the labels include the data value
• valueFormatter is a function to format the data value in the label. It's default is d3.format(",f") which just does comma thousand separators
• colourMap is a Map object which provides a category to colour lookup. This is used to override the categorical colour lookup, so you can do directional colours on the slope map.

I've also had so create a new wrapStyled function which wraps text elements with nested tspans for situations where you want one tspan to be bold like in the slope chart design.

Addresses #356

Please review for style @broadpeter, and for functionality anyone else.

[henryjameslau]

Ready for review @sam-ctrl

[nrennie]

I've fixed the conflicts and I'm happy with the code.

[nrennie]

@broadpeter Are you happy with the style on this template? Any changes?

[broadpeter]

Needs colour contrast boost to 1:4.5 on the regular weight text. I will try to add.
I also want to look at the leader lines label position