[Rule]: Create negative contractions rule

[claire-cheuk]

Is this

• a suggestion for a new rule
• an update to an existing rule
• a suggestion to remove a rule

Why is this change important

The style guide says not to use negative contractions

Examples of new behaviour as a result of the change

For example:

Scenario: Someone has used a negative contraction on a page (e.g. can't, won't)
Given a page exists
And the page contains a word that includes "n't"
When the linter runs against the page with the "no negative contractions" rule
Then the linter report should contain x errors
And the error message should contain "do not use negative contractions"

Add as many scenarios as you can think of to define the full behaviour of the rule. You can see existing examples in features/style_guide/*.feature

[cjfryer]

I think this is my favourite issue title of all time. Strong competition from #47 but I think this edges it.