feat: use distinct filter prompt titles

[ttytm]

#670

The titles are inspired by modern file mangers like dolphin. Which use "Filter" for live filtering. "Search" for seraching on submit.

Please let me know if any changes are desired or feel free to submit as co-auth.

[codecov]

Codecov Report

Merging #671 (1f4f76c) into main (3b41f0d) will decrease coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #671      +/-   ##
==========================================
- Coverage   49.58%   49.48%   -0.11%     
==========================================
  Files          47       47              
  Lines        6304     6309       +5     
==========================================
- Hits         3126     3122       -4     
- Misses       3178     3187       +9     

Impacted Files	Coverage Δ
lua/neo-tree/sources/filesystem/lib/filter.lua	11.11% <0.00%> (-0.50%)	⬇️
lua/neo-tree/sources/filesystem/init.lua	64.40% <0.00%> (-1.70%)	⬇️

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[cseickel]

Looks good. Thanks!

[ttytm]

Thanks for merging!

What I see now is that I probably would have named the variable popup_title if I hadn't been working with messages prior to this pr, and if the day hadn't already ended for my brain. But such semantic bean counting can be done at any time.