Lists are easier to scan when each column is alphabetically sorted vertically.
This is possible to do automagically with CSS columns, but Chrome doesn't like
CSS columns when links are using `text-decoration-thickness` [1][2] - so another
solution was needed.

CSS Grid Layout has support going back to IE 10[3] and allows for vertical
columns as long as the number of items in the columns are known - so when the
navigation is updated the CSS controlling the grid needs to be updated.

[1]: alphagov/govuk-frontend#2204
[2]: https://bugs.chromium.org/p/chromium/issues/detail?id=1190987
[3]: https://caniuse.com/css-grid

The mixin outputs the styles needed for the grid layout to work in IE10+ - this
is quite a repetative syntax, so the mixin takes the number of items and the
number of columns and works out the rest.

Added comments to the pertinent parts of the navigation localisation to remind
future people that they need to update the styles as well.