Improve imsc typing

@types/imsc/index.d.ts

package-lock.json

package.json

src/global.d.ts

src/routes/player/Player.tsx

src/routes/player/subtitles/SubtitleRenderer.tsx

src/routes/player/subtitles/TtmlHelper.ts

src/util/ttml/doc.js

src/util/ttml/html.js

src/util/ttml/isd.js

src/util/ttml/names.js

src/util/ttml/styles.js

src/util/ttml/utils.js

tsconfig.json