system/english_stenotype: improve RTF/CRE compatibility

[benoit-pierre]

Summary of changes

Support the arbitrary placement of the number sign when parsing steno.

Closes #1486.

Pull Request Checklist

• Changes have tests
• News fragment added in news.d. See documentation for details

[benoit-pierre]

Note: this behavior (arbitrary placement of the number sign) is conditional on a new optional system plugin flag (FERAL_NUMBER_KEY = True).

Rational:

• it's specific to the RTF/CRE spec, and so the English system
• consequently it does not make sense to have it as default for all systems
• it's also only possible if the number key is not an implicit hyphen key, and its corresponding letter is unique