feat: split output file into multiple smaller ones based on flag

[drgsn]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/core/output.go	82.29%	13 Missing and 4 partials ⚠️
cmd/filefusion/main.go	76.27%	9 Missing and 5 partials ⚠️

Files with missing lines	Coverage Δ
internal/core/types.go	92.98% <ø> (ø)
cmd/filefusion/main.go	72.47% <76.27%> (-4.66%)	⬇️
internal/core/output.go	85.65% <82.29%> (+0.73%)	⬆️