Fix 22 bugs, migrate to TypeScript, and modernise the toolchain

[emshotton]

Hey! I'm the original author of this library, I was delighted to see it was still being maintained and used after I left the BBC. Looks like it's been a few years since it's been updated, so I thought I'd have a go at revisiting it with Claude Code to see what could be fixed up. This PR does a bunch of modernization, bug fixing, and adding of new tests with only a relatively minor breaking change to the API (VideoContext.exceptions are now Error subclasses).

This ended up being a bit of a monster of a PR, and I'd be happy to split it into more reasonable sized chunks if preferred.

Summary

This PR does three related pieces of work, verified end-to-end at each step:

1. Fixes 22 bugs found in a full code review, each with a regression test
2. Converts src/ to strict TypeScript (compiled by Babel, type-checked by tsc)
3. Modernises the toolchain and packaging — Babel 7, Jest 29, ESLint 8, Prettier 3, TypeDoc, Node 20 CI, and dual UMD + ESM publishing with bundled .d.ts types

Tests grow from 61 to 125 (all passing); tsc --noEmit and ESLint are clean.

Bug fixes (each with a regression test)

API correctness

• playbackRate/volume setters now match nodes with instanceof MediaNode instead of a fragile constructor-name check, so they work on subclasses
• audio() now passes the media element cache (previously silently dropped)
• Deprecated createImageSourceNode forwards its arguments correctly
• New VideoContext.destroy() releases the update-loop registration and the global debug ref; reset() clears callbacks before the final update so stale UPDATE callbacks don't fire
• Added missing endOnLastSourceEnd getter; WebGL context creation falls back to experimental-webgl with the supplied context attributes

Render graph

• getInputsForNode places z-index connections on the port matching their index (previously order-dependent); out-of-range indexes fall back to the first free port
• Connecting to an unknown named port on an unlimited-input node now throws ConnectException instead of silently mis-connecting
• Exceptions are real Error subclasses (stack traces, instanceof works)

Source nodes

• Seeking while a node is waiting now updates its clock, so start(offset) is computed from the seeked time
• start()/startAt() refuse to re-sequence a destroyed node
• durationchange fires once when derived from the element, not on every frame for endless sources; media error handlers are detached on unload so they can't fire on dead nodes
• Ended nodes unload exactly once (MediaNode, and CanvasNode — found while writing coverage in the second commit)
• ImageNode uploads its static texture once instead of every frame, re-uploads after seeks, and handles user-supplied complete elements without racing onload

Processing nodes

• _render bound textures with the wrong texture-unit value (enum instead of index) — inputs beyond the first could sample the wrong texture
• destroy() deleted the wrong texture reference and leaked the vertex buffer

TypeScript & toolchain

• All 20 source modules + 21 effect/transition definitions converted to strict TS; no any escapes, honest casts are commented
• All 44 GLSL files inlined as template-literal constants (removes raw-loader and the per-definition package.json stubs)
• Babel 6 → 7 (@babel/preset-env + preset-typescript), Jest 23 → 29 (jsdom), ESLint 8 + @typescript-eslint, Prettier 3, JSDoc → TypeDoc, husky removed (CI is the gate), CI on Node 20 with a typecheck step
• Zero runtime dependencies (regenerator-runtime no longer needed)

Packaging

• main: UMD bundle (unchanged path), now Node-require safe (globalObject: "this")
• module + exports map: per-module ESM build (tree-shakeable, sideEffects: false)
• types: generated .d.ts declarations
• All four outputs (UMD, commonjs2, ESM, types) smoke-tested

Breaking / behavioural changes

• Browser support now follows browserslist "defaults" — IE11 is dropped
• Node ≥ 18 required to build; consumers are unaffected beyond the browser targets
• VideoContext.exceptions are now Error subclasses (message/stack behaviour changes if anyone matched on the old plain objects)

Not included (deliberately)

• Cypress 4 → 13 migration — the visual-regression suite runs unchanged
• cypress-image-snapshot replacement

Test plan

• 125/125 unit + integration tests pass (Jest 29, jsdom)
• tsc --noEmit clean (strict mode)
• ESLint clean
• UMD, commonjs2, ESM and .d.ts outputs built and smoke-tested
• Cypress visual-regression suite (unchanged; needs a maintainer run against the built bundle)