build: Use generated shaka namespace typedef in tests

[avelad]

Follow-up to #9596, which added a generated typedef for the shaka namespace to the externs but left the hand-maintained shakaNamespaceType typedef in place in the tests.

This generates a standalone externs file containing only the shaka namespace typedef and includes it when type-checking the tests, so the tests can refer to the whole library namespace in a type-safe way. The hand-maintained shakaNamespaceType typedef is then removed in favor of the generated shaka type.

Closes #6762

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[shaka-bot]

Bundle Size Report for PR #10288

File	HEAD	Base	Diff
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demo.compiled.debug.js	266.0 KiB (53.8 KiB)	266.0 KiB (53.8 KiB)	0.0 KiB
demo.compiled.js	266.0 KiB (53.8 KiB)	266.0 KiB (53.8 KiB)	0.0 KiB
demo.css	160.2 KiB (25.7 KiB)	160.2 KiB (25.7 KiB)	0.0 KiB
demo.modern.css	144.1 KiB (24.7 KiB)	144.1 KiB (24.7 KiB)	0.0 KiB
locales.js	44.5 KiB (11.0 KiB)	44.5 KiB (11.0 KiB)	0.0 KiB
receiver.compiled.debug.js	151.5 KiB (24.5 KiB)	151.5 KiB (24.5 KiB)	0.0 KiB
receiver.compiled.js	151.5 KiB (24.5 KiB)	151.5 KiB (24.5 KiB)	0.0 KiB
shaka-player.compiled-es2021.debug.js	1289.4 KiB (320.1 KiB)	1289.4 KiB (320.1 KiB)	0.0 KiB
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shaka-player.compiled.debug.js	1574.1 KiB (363.0 KiB)	1574.1 KiB (363.0 KiB)	+0.0 KiB
shaka-player.compiled.js	789.8 KiB (259.4 KiB)	789.8 KiB (259.4 KiB)	0.0 KiB
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shaka-player.hls-es2021.js	504.3 KiB (172.5 KiB)	504.3 KiB (172.5 KiB)	0.0 KiB
shaka-player.hls.debug.js	1190.9 KiB (277.9 KiB)	1190.9 KiB (277.9 KiB)	+0.0 KiB
shaka-player.hls.js	585.9 KiB (195.2 KiB)	585.9 KiB (195.2 KiB)	0.0 KiB
shaka-player.transmuxer-worker.debug.js	191.8 KiB (46.9 KiB)	191.8 KiB (46.9 KiB)	-0.0 KiB
shaka-player.transmuxer-worker.js	83.6 KiB (28.5 KiB)	83.6 KiB (28.5 KiB)	0.0 KiB
shaka-player.ui-es2021.debug.js	1598.9 KiB (389.4 KiB)	1598.9 KiB (389.4 KiB)	0.0 KiB
shaka-player.ui-es2021.js	896.0 KiB (289.2 KiB)	896.0 KiB (289.2 KiB)	0.0 KiB
shaka-player.ui.debug.js	1920.5 KiB (437.8 KiB)	1920.5 KiB (437.8 KiB)	+0.0 KiB
shaka-player.ui.js	1016.6 KiB (322.5 KiB)	1016.6 KiB (322.5 KiB)	0.0 KiB

[shaka-bot]

Incremental code coverage: No instrumented code was changed.

[avelad]

@joeyparrish @TAhub can you review it? Thanks!