Support pwsh format for show-env export

[LittleBoxOfSunshine]

This PR demonstrates a solution for #395, with a slightly different strategy that what was discussed.

The issues outlined there:

1. No pre-built in option for ouputing PowerShell syntax
2. PowerShell has very different (and frankly fairly obtuse) escaping rules which is an impediment for 1. Specifically, env vars like CARGO_ENCODED_RUSTFLAGS take non-printable characters that unix shells will accept but pwsh won't.

Additionally, the shell escape crate used is escaping for cmd, not pwsh and wouldn't be compatible.

Pwsh does accept embedded unicode codes in strings though. If we encode the values this way, we don't need to worry about printable vs non-printable characters nor any of the specifics for what kinds of characters need to be escaped for what kinds of strings. This is a one-time, very fast operation. Once the emitted string is executed all the escapes are gone.

This approach wouldn't be good for a general-purpose escape function, but it's perfectly suitable for this use case.

The change enables users to execute a pwsh version of the source demo:

Invoke-Expression (cargo llvm-cov show-env --export-pwsh-prefix | Out-String)

Primary goal was to confirm this approach worked. If this is of interest, I can make any changes to the code organization, arg name, etc. The current organization is just what came to mind as I played around with the repo.

Manual test context with binary data in env:

Intermediate state example:

Final result:

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Closes #395

[asvrada]

Thanks for this!

Confirmed reading as Unicode doesn't work with PowerShell Version 5.1. But works after v6.0.0.0

Other than that, it looks good.

[taiki-e]

Thanks!

[taiki-e]

Published in 0.6.16.