Add NSIS custom installer script for UAC fix on Windows

[vgrassia]

Resolves https://bitwarden.atlassian.net/browse/PS-792

Type of change

- [X] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

This PR fixes our Windows installer from removing Bitwarden and crashing before installing the new version.

Found the fix here: electron-userland/electron-builder#2363 (comment)

Code changes

• apps/desktop/electron-builder.json: Adds the include key to our nsis config to specify the custom installer script
• apps/desktop/installer.nsh: Custom installer script that fixes the issue of prompting for UAC when Bitwarden is installed for All Users

Before you submit

- [ ] I have checked for **linting** errors (`npm run lint`) (required)
- [ ] I have added **unit tests** where it makes sense to do so (encouraged but not required)
- [ ] This change requires a **documentation update** (notify the documentation team)
- [ ] This change has particular **deployment requirements** (notify the DevOps team)