feat: implement the env declaration modifier in WDL 1.2.

[peterhuene]

This PR implements the env declaration modifier in WDL 1.2 that may be present for both inputs and private declarations in a task.

When present, task execution will treat the value of the declaration as an environment variable for the spawned process. The variable can then be referenced using Bash syntax (e.g. $foo).

Validation diagnostics were added to ensure the env variable is only applied to primitive type declarations, as there is no standard string representation for complex types.

Closes #295.

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[a-frantz]

wdl-format needs to be updated as well. Or else it will panic on anything with the env keyword.

[peterhuene]

I'll add a test case to wdl-format and update it.

[claymcleod]

LGTM.