Refactor to stop using FZF_DEFAULT_COMMAND

[ghost]

Changes

• Stop using FZF_DEFAULT_COMMAND in favor of piping output into fzf.
  • Previously many functions set this environment variable prior to executing FZF for non-standard uses. This was required when make-term was running fzf directly, but now that we are executing sh -c and passing args, we can pipe output of any command into fzf.
• Update fzf/start to optionally take a cmd which is then piped into fzf. If no cmd is provided, fzf runs with whatever FZF_DEFAULT_COMMAND is (this is the same as before)
• Add comments to after-term... and slight refactor to remove redundant code (no functional change)

[bling]

can you please remove all the formatting changes? this creates a bunch of unnecessary diffs to review. also, by convention the fzf/ prefix is meant for private variables/functions, whereas fzf- is the public API meant to be called externally.

this dovetails into fzf-with-command. instead of deleting it and potentially breaking a ton of people out there, it'd be better to keep it around, call fzf/start internally and output a deprecation message so we can safely delete it in the future.

for the remaining changes....these are pretty major. perhaps @rolag has some thoughts here as they contributed the completion engine.

[ghost]

Hey @bling thanks for taking a look

Yea, sorry about that. I trimmed this PR down a lot to only include the bare minimum changes required to stop setting the environment variable, and a slight refactor to the after-term... function to help readability. Hopefully this makes the important changes clearer.

I'll hold the configurable grep change for now to keep prs incremental.