add simple head/tail modifiers

[josharian]

Checklist

• I have added tests
• [-] I have updated the docs and cheatsheet
• [-] I have not broken the cheatsheet

[josharian]

cc @pokey -- not for review yet, a 📌 for a conversation for a possibly terrible idea i had about handling non-simple head/tail

[pokey]

I believe we discussed this and decided that it would be better to have one token to indicate the start of a head or tail modifier and one token to indicate the end. If nothing falls in between then it is a head or tail of line, otherwise it's a head or tail of whatever modifier sequence falls in between. Does that line up with your recollection?