In the function check_placeholder in wdl-analysis, we're accepting arrays of primitives for the sep option, but otherwise not enforcing the requirements from the WDL specification relating to placeholder options.

For the sep option, it is only allowed if the type of the expression is Array[P] (where P is any primitive type).

For the default option, it is only allowed if the type of the expression is optional.

For the true and false option, it is only allowed if the type of the expression is Boolean.

We should emit diagnostics when the types of the expressions are not the expected one for the option.

Additionally, in wdl-engine, we should also be enforcing these requirements and emitting the same diagnostic if the value produced by the expression is of a type that is not supported by the placeholder option.

This is needed as the type calculation for an expression in wdl-analysis can be Union, which means the type is statically indeterminate. An example of this would be ~{true="yes" false="no" read_json("file")}. This should emit no diagnostics from wdl-analysis, but should emit a diagnostic error from wdl-engine if the file does not contain a boolean.