feat: density of a union of finsets

[YaelDillies]

From PlainCombi (LeanCamCombi)

---

[github-actions]

PR summary eff25e81ec

Import changes exceeding 2%

%	File
+18.26%	Mathlib.Algebra.BigOperators.Field

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Algebra.BigOperators.Field	586	693	+107 (+18.26%)

Import changes for all files

Files	Import difference
Mathlib.Combinatorics.SetFamily.LYM	15
Mathlib.Algebra.BigOperators.Field	107

---

Declarations diff

+ dens_biUnion
+ dens_biUnion_le
+ dens_disjiUnion
+ dens_eq_sum_dens_fiberwise
+ dens_eq_sum_dens_image

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[b-mehta]

Looks great! I'm content with the large-import since there's a clear (and documented) plan to resolve it in the future. Thank you!