textinput autocomplete - suggestions at any position

**Is your feature request related to a problem? Please describe.**
No.

**Context**
I'm building a TUI that reaches out to a code search engine. The backend engine accepts tokens for modifying the query, like `file:, repo:, archived:yes|no,` etc. Some tokens can be repeated, and tokens can be included anywhere in the query. For example, `hello file:.hs` or `file:.hs hello`.

The new suggestions implementation (tks @toadle !) only displays suggestions at the 0th offset, e.g, only at the start of input. Once the first word has been typed, suggestions no longer appear.

**Describe the solution you'd like**

I'd like to display suggestions at any position within the text input.

I'd also like to be able to mark suggestions as repeatable/or-not. This is much less important, and complicates the code a fair amount.

**Describe alternatives you've considered**
None, really.

**I get how hard maintaining/implementing open source feature reqs is, so I'm going to try this on my own and then come back and see what parts, if/any, you all would like to port back in.**