Plugin validator should support nunjucks functions

[BenSurgisonGDS]

The plugins validator should allow nunjucks functions to be included by validating the prototype config key nunjucksFunctions

[colinrotherham]

Looks good, seems to run the checkPathExists() validation still

Would be great to maintain a JSDoc definition so we don't lose track of fields like this again in future? There's one I made already over in 39a3b41 which can help catch issues too

Otherwise the schema comment in plugins.js is very out of date:

govuk-prototype-kit/lib/plugins/plugins.js

Lines 14 to 30 in 1fc78df

	* A schema for an example manifest file follows:
	*
	* // govuk-prototype-kit.config.json
	* {
	* "assets": path | path[],
	* "importNunjucksMacrosInto": path | path[],
	* "nunjucksPaths": path | path[],
	* "nunjucksFilters": path | path[],
	* "sass": path | path[],
	* "scripts": path | path[],
	* "stylesheets": path | path[],
	* "templates": {
	* "name": string,
	* "path": path,
	* "type": string
	* }[]
	* }

[BenSurgisonGDS]

Looks good, seems to run the checkPathExists() validation still

Would be great to maintain a JSDoc definition so we don't lose track of fields like this again in future? There's one I made already over in 39a3b41 which can help catch issues too

Otherwise the schema comment in plugins.js is very out of date:

govuk-prototype-kit/lib/plugins/plugins.js

Lines 14 to 30 in 1fc78df

	* A schema for an example manifest file follows:
	*
	* // govuk-prototype-kit.config.json
	* {
	* "assets": path | path[],
	* "importNunjucksMacrosInto": path | path[],
	* "nunjucksPaths": path | path[],
	* "nunjucksFilters": path | path[],
	* "sass": path | path[],
	* "scripts": path | path[],
	* "stylesheets": path | path[],
	* "templates": {
	* "name": string,
	* "path": path,
	* "type": string
	* }[]
	* }

Thank you for spotting this. I'll update this in a separate PR