[Icons] Export icon constants

[sorenlouv]

Currently to use a (third party) icon it is only possible to use a string as the logo identifier:

<EuiIcon type='logoAWS' />

This is error prone since the there's nothing preventing the dev making a spelling mistakes, or renaming an icon on the EUI side without updating the consumers.
To prevent these mistakes, catch changes and improve intellisense I suggest icons are consumed as constants:

<EuiIcon type={euiIcons.logoAws} />

Afaict all that's needed is to export typeToPathMap:

eui/src/components/icon/icon.tsx

Lines 39 to 56 in e475ce5

	const typeToPathMap = {
	accessibility: 'accessibility',
	addDataApp: 'app_add_data',
	advancedSettingsApp: 'app_advanced_settings',
	aggregate: 'aggregate',
	alert: 'alert',
	analyzeEvent: 'analyze_event',
	annotation: 'annotation',
	apmApp: 'app_apm',
	apmTrace: 'apm_trace',
	apps: 'apps',
	appSearchApp: 'app_app_search',
	arrowDown: 'arrow_down',
	arrowLeft: 'arrow_left',
	arrowRight: 'arrow_right',
	arrowUp: 'arrow_up',
	asterisk: 'asterisk',
	auditbeatApp: 'app_auditbeat',

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