Add an automated test suite + CI pipeline

[MBuelowius]

Summary

PptxGenJS is widely used in production, but the repo currently has no automated test suite or CI — testing is manual per TESTING.md (render the demos and eyeball the output). This makes regressions in the XML-generation paths easy to miss.

Proposal

A lightweight, low-dependency setup:

• Runner: Node's built-in node:test + tsx (run the TS sources directly — no Jest/Vitest, no extra config).
• Unit tests for the pure helpers in src/gen-utils.ts (EMU/point conversions, color + XML-escaping helpers, shadow-option normalization).
• End-to-end test that builds a deck (text + shape + table + chart), writes it to a buffer, and asserts the resulting .pptx is a valid OOXML package: required parts present, every XML part well-formed, expected content present. This exercises gen-xml / gen-objects / gen-tables / gen-charts together.
• CI (GitHub Actions): lint + typecheck + test + build on Node 20 & 22.
• Optional extras: a prepublishOnly gate, Dependabot, CodeQL.

package.json would gain lint / typecheck / test scripts (eslint is already configured, just not wired to a script).

Offer

I've prototyped all of the above (18 passing tests, including the OOXML validation) in a fork. Happy to open a PR if this is welcome — glad to adjust the runner or scope to your preference.