Unhide strikethrough field type in template builder

The strikethrough field type is fully implemented end-to-end (rendering,
non-editable enforcement, audit trail, i18n in 14 languages) but filtered
out of the default field picker by the skipTypes array in both
field_type.vue and fields.vue. Without that filter, end-users have no way
to discover the feature short of reading source.

Removing strikethrough from skipTypes lets it appear in the picker
alongside other field types. The other two filtered types (heading,
datenow) remain hidden as before.

Use case: counter-offer markup workflows where the template author
strikes through clauses they are countering and places initials boxes
nearby for the signer to acknowledge each modification. The strikethrough
is author-only and non-editable by signers (already enforced in
NONEDITABLE_FIELD_TYPES at submit_values.rb:10) — perfect fit, just
needs to be visible.

app/javascript/template_builder/field_type.vue

@@ -164,7 +164,7 @@ export default {
       }
     },
     skipTypes () {
-      return ['heading', 'datenow', 'strikethrough']
+      return ['heading', 'datenow']
     },
     fieldIconsSorted () {
       if (this.fieldTypes.length) {

app/javascript/template_builder/fields.vue

@@ -574,7 +574,7 @@ export default {
       }, {})
     },
     skipTypes () {
-      return ['heading', 'datenow', 'strikethrough']
+      return ['heading', 'datenow']
     },
     fieldIconsSorted () {
       if (this.fieldTypes.length) {