Adding a test for multiple spaces in sentences

GitLab's style guide specifies a single space between sentences,
and this test checks to make sure the style guide is being
followed.

doc/.linting/vale/styles/gitlab/SentenceSpacing.yml

+---
+# `extends` indicates the Vale extension point being used.
+# Full list of styles: https://errata-ai.github.io/vale/styles/
+extends: existence
+
+# Existence rules can display the matched strings in the user message.
+message: "'%s' should have one space between sentences."
+
+# Should a result be flagged as a suggestion, warning, or error?
+# Results that fall below the MinAlertLevel set in 
+# https://gitlab.com/gitlab-org/gitlab/blob/master/.vale.ini won't be shown.
+level: suggestion
+
+# Should a match be case-insensitive or case-sensitive?
+# Acceptable values are 'true' or 'false'
+# This value is irrelevant when testing non-alphabetical characters
+#ignorecase: true
+
+# Should this rule be limited to a specific scope? If yes, uncomment the line.
+# Possible scopes: https://errata-ai.github.io/vale/formats/#available-scopes
+# scope: heading
+
+# Should this rule ignore normal word boundaries, such as \b ?
+# Acceptable values are 'true' or 'false'
+nonword: true
+
+# What is the source for this rule?
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#punctuation
+
+tokens:
+  - '[a-z][.?!][A-Z]'
+  - '[.?!] {2,}[A-Z]'