Add rule: declarations before nesting

[onishiweb]

What do the changes you have made achieve?
Added new rule to lint for declarations appearing after nested rulesets

Are there any new warning messages?
Yes: "Declarations should come before nestings"

Have you written tests?
Yes

Have you included relevant documentation
Yes

Which issues does this resolve?
Closes #865

<DCO 1.1 Signed-off-by: Adam Onishi adam.onishi@ft.com>

[coveralls]

Coverage increased (+0.02%) to 97.549% when pulling 9a7b528 on onishiweb:feature/declarations-before-nesting into b7bcb3a on sasstools:develop.

[coveralls]

Coverage increased (+0.02%) to 97.549% when pulling 9d671c4 on onishiweb:feature/declarations-before-nesting into b7bcb3a on sasstools:develop.

[coveralls]

Coverage increased (+0.02%) to 97.549% when pulling 9d671c4 on onishiweb:feature/declarations-before-nesting into b7bcb3a on sasstools:develop.

[coveralls]

Coverage increased (+0.02%) to 97.549% when pulling 9d671c4 on onishiweb:feature/declarations-before-nesting into b7bcb3a on sasstools:develop.

[coveralls]

Coverage increased (+0.02%) to 97.549% when pulling 9d671c4 on onishiweb:feature/declarations-before-nesting into b7bcb3a on sasstools:develop.

[bgriffith]

Thanks for your PR @onishiweb - this looks great 👍 Could you update it with the latest from develop?

[coveralls]

Coverage increased (+0.02%) to 97.563% when pulling 8a2168f on onishiweb:feature/declarations-before-nesting into 977f5a4 on sasstools:develop.

[onishiweb]

Thanks @bgriffith :) Also thanks to @JakeChampion for the help with the test :)