Split apart contraction rules

Split contraction rules into two files to make it possible to
use different messages for contractions we keep, vs contractions we
discard.

doc/.vale/gitlab/Contractions.yml → doc/.vale/gitlab/ContractionsDiscard.yml

 ---
-# Suggestion: gitlab.Contractions
+# Suggestion: gitlab.ContractionsDiscard
 #
-# Checks for use of common and uncommon contractions.
+# Suggests a list of agreed-upon contractions to discard.
 #
 # For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
 extends: substitution
@@ -12,18 +12,6 @@ nonword: false
 ignorecase: true
 swap:
 
-  # Common contractions are ok
-  it is: it's
-  can not: can't
-  cannot: can't
-  do not: don't
-  have not: haven't
-  that is: that's
-  we are: we're
-  would not: wouldn't
-  you are: you're
-  you have: you've
-
   # Uncommon contractions are not ok
   aren't: are not
   couldn't: could not

doc/.vale/gitlab/ContractionsKeep.yml

+---
+# Suggestion: gitlab.ContractionsKeep
+#
+# Suggests a list of agreed-upon contractions to keep.
+#
+# For a list of all options, see https://errata-ai.gitbook.io/vale/getting-started/styles
+extends: substitution
+message: 'Use "%s" instead of "%s", for a friendly, informal tone.'
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#language
+level: suggestion
+nonword: false
+ignorecase: true
+swap:
+
+  # Common contractions are ok
+  it is: it's
+  can not: can't
+  cannot: can't
+  do not: don't
+  have not: haven't
+  that is: that's
+  we are: we're
+  would not: wouldn't
+  you are: you're
+  you have: you've