Add vale rule to check for empty image alt tags (#3034)

Author: markzegarelli

.github/styles/segment/alt-tag.yml

@@ -0,0 +1,7 @@
+extends: existence
+message: 'Please add alt text to the image'
+level: warning
+scope: raw
+nonword: true
+raw:
+  - '(\!\[\])'

src/utils/vale-test.md

@@ -11,5 +11,4 @@ For example, like [this](/docs/connections)
 segment.com/docs
 
 Here's an update.
-
-
+![alt tag](/diagram-library/Elements.png)

vale-styles/segment/alt-tag.yml

@@ -0,0 +1,7 @@
+extends: existence
+message: 'Please add alt text to the image'
+level: warning
+scope: raw
+nonword: true
+raw:
+  - '(\!\[\])'