chore: migrate code-snippet

[jacob-kreyenbuehl]

Summary

Migrates the Code Snippet framework from SAP/code-snippet into the app-studio-toolkit monorepo under projects/code-snippet.

Key changes

• Adds the public @sap-devx/code-snippet-types package plus private backend, frontend, wrapper, and example workspaces.
• Wires the backend and frontend as workspace build dependencies and adds the backend's missing runtime dependencies.
• Adds the fixed/ignore changeset configuration, legacy TypeScript and ESLint compatibility, pnpm-safe VSIX packaging, and monorepo metadata.
• Preserves historic changelogs as CHANGELOG.old.md and updates project documentation and package links.

Behaviour

No changeset is included because this is a relocation-only migration. The maintenance-release changeset will be created after this PR merges. Scoped package CI, Prettier, and ESLint are green.

Reviewer's guide

95 files changed, but most are a source lift. Review in this order:

1. Release wiring: .changeset/config.json, workspace discovery, package metadata, and the public types package.
2. Build and lint configuration: tsconfig.base.json, root ESLint override, nyc/mocha scripts, webpack, and VSIX ignore files.
3. Dependency wiring: backend runtime dependencies and the frontend/types workspace:* relationships.
4. Everything else: skim as a formatted source migration.

Not in this PR (deliberately): no changeset or release/version bump. @sap-devx/code-snippet-types already exists on npm at 2.0.0; its maintenance release must be handled in the post-merge changeset PR.