Add Nunjucks render helpers from forms designer

[colinrotherham]

This PR brings over the Nunjucks helpers we use on forms-designer

import * as render from '~/src/server/plugins/nunjucks/render.js'

render.macro()
render.string()
render.view()

I've used it as a good opportunity to migrate the views plugin from TypeScript to plain ES modules

Nunjucks within view models

Why is this helpful?

1. Nunjucks views need tests too
2. Nunjucks strings and macros often need evaluating earlier

For example, see where we safely encode notification emails in forms-designer using:

render.string(
  'Completed forms will be sent to <a href="mailto:{{ notificationEmail | urlencode }}" class="govuk-link">{{ notificationEmail }}</a>.',
  { context: form }
)

Or alternatively, rendering a macro without string-building raw HTML

render.macro('summaryValue', 'partials/summary-value.html', {
  params: item
})

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud