Allow Nunjucks component params.attributes to support strings

[colinrotherham]

Related component

All components currently render Nunjucks attributes by looping object name/value pairs:

{%- for attribute, value in params.attributes %} {{ attribute }}="{{ value }}"{% endfor %}

This is unlike params.classes where string values are uses instead:

class="govuk-table__header {%- if item.classes %} {{ item.classes }}{% endif %}"

But this has prevented us from appending or merging new formGroup attributes to existing defaults in #4566 and in Create Password spikes #4512 (comment)

Context

This feature request comes from a by-design Nunjucks snag

Like in Jinja, object modification is not possible mozilla/nunjucks#307

Whilst looping object name/value pairs give us more flexibility (filtering, sorting), this means:

• Individual attributes cannot be added
• Individual attributes cannot be removed
• Individual attributes cannot be reordered

This has been mentioned in the past (see #995, #1529, and mozilla/nunjucks#1455 etc)

The suggestion to support string values came from @36degrees in #4566 (comment)

Alternatives

Alternatively, we've tried a clever hack to push to Object.entries() arrays in Nunjucks for loops

But it's quite obscure and arrays lack named property access used by govukI18nAttributes() etc

Attributes as plain object

{{ govukButton({
  text: "Press me",
  attributes: {
    data-example: "value"
  }
}) }}

Attributes as Object.entries() array

{{ govukButton({
  text: "Press me",
  attributes: [
    ["data-example": "value"]
  ]
}) }}

Additional information (if applicable)

Supporting params.attributes as strings may simplify work on proposed attribute macros and we may want params.classes to work with string or object types too.

[colinrotherham]

Up for review in #4742