Export issue and issueCommand from core

[kimburgess]

Describe the enhancement
The core package defines the very useful issueCommand and issue functions, however these are currently available for internal use only.

Code Snippet
Exporting these would allow common usage scenarios such as

core.info(`::add-matcher::${Path.join(some_base_path, 'foo.json')}`)

(or worse)

To be more cleanly expressed as

core.issue('add-matcher', Path.join(some_base_path, 'foo.json'))

Additional information
This will also isolate usage from any changes to the underlying command structure (see #110), as well as ensuring any required escaping is applied.