Implement deprecation of @ember/string per RFC

[kategengler]

I'm attempting to implement the long-ago RFC'd deprecation of the internal @ember/string package in place of the published addon @ember/string.

This issue is to summarize what I find:

There was an aborted attempt to deprecate import { htmlSafe, isHTMLSafe } from '@ember/string' (with the new import being import { htmlSafe, isHTMLSafe } from @ember/template`).

Current status, on a newly generated app on 4.9.3:

import { htmlSafe } from `@ember/string`;
htmlSafe('foo'); // errors (0, _string.htmlSafe) is not a function

import { htmlSafe } from `@ember/template`;
htmlSafe('foo'); // success

On a newly generated app on 4.9.3, with ember-data removed:

import { htmlSafe } from `@ember/string`;
htmlSafe('foo') // success

import { htmlSafe } from `@ember/template`;
htmlSafe('foo'); // success

If the addon @ember/string is included, it clobbers the internal @ember/string. To solve this, htmlSafe and isHTMLSafe should be added to the addon @ember/string with deprecations pointing to the imports from @ember/template. I have confirmed that the deprecations, when added back to the internal @ember/string, do not trigger when using the @ember/template imports (this was the original issue that led to the deprecation being backed out).

The htmlSafe/isHTMLSafe import from @ember/template was added in 3.8.

Todo

• Add htmlSafe and isHTMLSafe to the addon @ember/string with errors pointing to the imports from @ember/template. Add htmlSafe and isHTMLSafe exported methods. These will error when called, telling the user to import from @ember/template. ember-string#367
• Re-enable deprecations for importing htmlSafe and isHTMLSafe from @ember/string. They have moved to @ember/template. #20341
• Ember.String.htmlSafe (and all other methods) are still available on Ember when doing import Ember from 'ember'. Deprecate accessing String on the global import. Deprecate @ember/string when used from ember-source; point users to add the @ember/string addon #20344
• Deprecate the methods on internal @ember/string, tell users to add addon Deprecate @ember/string when used from ember-source; point users to add the @ember/string addon #20344
• Ember.STRINGSDeprecate @ember/string when used from ember-source; point users to add the @ember/string addon #20344

[kategengler]

This is complete in ember-source