feat: add support for listing several files at once via curly brace expressions

[pursvir]

For example:

todo.sh lf {inbox.txt,maybe.txt} @calls John
todo.sh lf archive/done-{2025,2026}.txt Finish

In my opinion, this will highly improve UX when there are multi-file setups.

[inkarkat]

What's the use case for multi-file setups? To me, that's rather an anti-pattern, as todo.txt has all the filtering (by project, context) to manage any tasks in a central place. There have been requests to separate work and private tasks, and that usually is achieved by having two different config files (which allow other differentiations as well), and aliases to automatically choose one or the other. But that separation would be a counter-argument to having commands that cover several files at once.

Some people want to use per-project todo files (cp. #187; I'm also experimenting with that (todo-local.sh)), but again the intention is separation, not aggregation (and the distribution across entire directory hierarchies would make a curly brace expansion or any form of shell globbing much less useful, in addition to listfile current only supporting files from the TODO_DIR itself).

Speaking of UX, a huge challenge would be how to present the different sources. todo.txt is based on the implicit file name, and task manipulation is through task NR; i.e. the line numbers. With multiple files, the bare line numbers would be meaningless. I think listfile was added as a convenience command, to quickly query done.txt, but never with the intention to support multiple active files. For that, users would expect all built-in commands to support different target files, and that would be a huge change. The solution of multiple config files and different todo aliases is much more elegant in that respect, even though it doesn't support any form of aggregation.

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todo.sh lf archive/done-{2025,2026}.txt Finish

Is there really a need to split the archive by year? My done.txt is ~1 MB in size, with 7000 tasks covering 16 years. That's still very manageable. Filtering for date ranges can be implemented much more flexibly with a custom TODOTXT_FINAL_FILTER.