fix: removes struct, task, and workflow from keywords

These should only be highlighted in the context of struct, task, and
workflow declarations—things like `import @stjudecloud/workflow`
(a phrase that I was tested out as an importing paradigm) should not
highlight the word 'workflow'.

syntaxes/wdl.tmGrammar.json

@@ -390,15 +390,15 @@
         },
         {
           "name": "storage.type",
-          "match": "\\b(command|hints|inputs|meta|object|outputs|parameter_meta|requirements|runtime|struct|task|workflow)\\b\\s*(?!:)"
+          "match": "\\b(command|hints|inputs|meta|object|outputs|parameter_meta|requirements|runtime)\\b\\s*(?!:)"
         },
         {
           "name": "constant.language.wdl",
           "match": "\\b(true|false|left|right|null)\\b"
         },
         {
           "name": "keyword.wdl",
-          "match": "\\b(after|alias|as|call|command|else|false|hints|if|in|import|input|meta|null|object|output|parameter_meta|requirements|runtime|scatter|struct|task|then|true|version|workflow)\\b\\s*(?!:)"
+          "match": "\\b(after|alias|as|call|command|else|false|hints|if|in|import|input|meta|null|object|output|parameter_meta|requirements|runtime|scatter|then|true|version)\\b\\s*(?!:)"
         },
         {
           "name": "entity.name.type.wdl",