feat: add escrow release milestone wrapper

[MyouzzZ]

Summary

• add ReleaseMilestoneParams and buildReleaseMilestoneArgs for the Soroban release_milestone call
• add and export releaseMilestone() with escrow id, milestone id, amount, and caller validation
• add Jest coverage for the generated ScVal/XDR payload order and invalid release milestone params

Closes #11

Validation

• npm test -- --runTestsByPath tests/escrow.test.ts
• npm run build
• npm run lint exits 0 with existing repository warnings

Notes

• Local dependency install used npm install --ignore-scripts because the checked-in lockfile is currently missing npm optional platform entries required by npm ci; no lockfile changes are included in this PR.