Create new OOUI widget for searching templates, with the same functionality as the merger of the existing template search fields in VisualEditor and TemplateWizard (i.e. CirrusSearch if available, prefix search if not).
TemplateWizard searches with a `prefixsearch` generator:
```
action=templatedata
includeMissingTitles=1
generator=prefixsearch
gpssearch=mytpl
gpsnamespace=10
gpslimit=10
redirects=1
```
Whereas VE uses a `search` generator (in addition to prefixsearch; clarity needed about why it makes the prefixsearch request):
```
action=query
prop=info%7Cpageprops
generator=search
ppprop=disambiguation
redirects=true
gsrsearch=mytpl*
gsrnamespace=10
gsrlimit=10
gsrprop=redirecttitle
``
```
action=query
format=json
formatversion=2
generator=prefixsearch
gpssearch=mytpl
gpsnamespace=10
gpslimit=10
```
Then with the results of that, it fetches the templatedata:
```
action=templatedata
titles=Template%3ATemplate1%7CTemplate%3ATemplate2 (lots more here…)
includeMissingTitles=1
```
Neither search input debounces, but rather send API requests on every keystroke. TemplateWizard aborts on subsequent ones; VisualEditor doesn't.