Simplify customisation of line height

Added new variable `@line-height-target` with a default value to `20px` to match the existing style.
Then updated `@line-height-base` so it was no longer a magic number with a comment of `20 / 14`

Seeing as this value was using math I've made it more explicit. The `@line-height-base` is now a unitless value derived from the base font size and new line height target.

Seeing as now the `@line-height-base` is variable driven I removed it from the customiser along with`@line-height-computed`
I think this may make it easier for people to customise the variables file and get the values they want without have to do the math themselves, and seems a lot more explicit.

Note: Would this now make `@line-height-computed` redundant? I've kept it it for compatibility but could this variable become deprecated in a future release?