btxe parse b:t > b:on only handles explicit characters in suffix parsing

[garethjenkins]

consider whether this should allow any suffix chars and punctuation (up to two chars)

[crebello]

Other instances:

• speech marks.
• currently working with algebraic expressions, so would like to be able to write "3f" or "6x + 2" etc. Don't know whether we are planning any features that will make this request redundant?