Issue #5286: Add some words that start with "un"

Fixes streetsidesoftware/cspell#5286

CSpell has been nagging about these words found in the Backdrop CMS codebase.

dictionaries/en_US/src/additional_words.txt

@@ -40,6 +40,7 @@ guillemet
 heatmap
 hmmm
 infographic
+instantiatable
 iPads
 Kyiv
 multiline
@@ -54,12 +55,32 @@ rebrand
 rebranding
 teleconverter
 tricep
+unaggregated
+uncustomized
+undeletable
+unescapes
+unescaping
+unextracted
+unflag
+unflagged
 ungrouped
+unhighlight
+unhighlights
+uninstantiatable
 unintuitive
 unmaneuverable
+unmunge
+unmunged
+unmunging
+unnest
+unnested
+unnesting
+unpackage
+unpackaged
 unparsable
 unpatched
 unplated
+unrendered
 unsecure
 unsecured
 unsecures