As part of future changes, we need to be able to specify on a
per-command level whether a `shell` should be used.
Before we do this, we should make it possible to provide the `shell`
specifier.
As part of future changes which will fix issues with Composer + Yarn 1
introduced by f430552de4, we need to find
a way to ignore command failure, on a per-command basis.
To do this, we can provide a refactor to introduce a new type,
`CommandWithOptions` which has the ability to specify whether to
`ignoreFailure`.
To make this less of a refactor than would otherwise require, we can
pass around a `(string | CommandWithOptions)[]`
(or a `string[]` or a `CommandWithOptions[]`) to be used, and use
helpers `asRawCommand` and `asRawCommands` to extract the underlying
command where necessary.
In the case a command's failure is being ignored, return the error code
in the `ExecResult`.
We also need to make sure that when executing in Docker, we still
provide the `|| true`s, otherwise we'll start to see commands failing
unexpectedly.
As part of future-proofing the `CommandWithOptions`, we can make sure
that it uses `string[]`s for the command it's representing, to avoid
re-parsing it.
Sergei Zharinov