feat: Consider creating function pairs: `assert_*` and `is_*`

[jdhoffa]

          I have been questioning whether there should be both `assert_valid_*()` and `is_valid_*()` style functions for each data type it knows, one that errors or returns TRUE invisibly and one that returns TRUE or FALSE. That would be a rather large "interface" change. There are also a few significant things missing.

Originally posted by @cjyetman in #73 (comment)

AB#10915