Support block based image banner pixels

[philwebb]

With #18264 merged, many image banners would look better with unicode block chars rather than letters. It might be nice to offer this as an option.

[wilkinsona]

The test introduced in 303974f is failing on Concourse:

08:54:33 [ERROR]   ImageBannerTests.printBannerWhenPixelModeIsBlockShouldRenderBlocks:187 
08:54:33 Expecting:
08:54:33  <"
08:54:33
08:54:33 ????? 
08:54:33
08:54:33 ">
08:54:33 to contain:
08:54:33  <"???? ">

I don't know why yet. It passes locally on macOS and Linux.

[wilkinsona]

It's an encoding problem. On Concourse or in the CI Docker image run locally, the StreamEncoder beneath the PrintStream uses US-ASCII encoding. When run locally it's using UTF-8. However, setting the encoding on the PrintStream created in the tests doesn't appear to be sufficient to fix the problem.