age: Replace atty with is-terminal

[str4d]

Closes #359.

[codecov]

Codecov Report

Merging #405 (4d81bb0) into main (3b29ef6) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
- Coverage   40.91%   40.88%   -0.04%     
==========================================
  Files          34       34              
  Lines        3248     3248              
==========================================
- Hits         1329     1328       -1     
- Misses       1919     1920       +1     

Files Changed	Coverage Δ
age/src/cli_common/file_io.rs	16.12% <0.00%> (ø)

... and 4 files with indirect coverage changes