Add initial set support

[Neved4]

Fixes #34 #35

Would love some guidance to create the tests.

Note: Currently doesn't support set -- x y "$@" to append. This is equivalent to fish's set -a argv x y

[bouk]

Nice, just some tests :)

[Neved4]

Done!

[bouk]

Now we're just silently ignoring this, maybe that's not good? Can we support some of these options? Or is there no fish equivalent?

[Neved4]

I don't think there's any fish equivalent for these, see the picture in: #34 (comment).

And the later description:

Of these, -C doesn't exist in fish, and -e, -f, -u all have different meanings.

As-is they'll break fish scripts, so my approach is to filter them out.

For more detail, someone writing set -e in UNIX shells is expecting to terminate the script early on errors, with some caveats. Someone writing set -e on fish is expecting to erase a shell variable, akin to what would be unset.

[Neved4]

I'm yet to support set - x as opposed to set -- x.

set -- x "$@" into set -a argv x looks more challenging, but I'll still give it a try.

Makes sense to add those two translations to this PR.

Checklist:

• set - x -> set argv x
• set -- x -> set argv x
• set -- x "$@" -> set -a argv x
• set -Cefu -> , empty string

[Neved4]

Supported set -x, but stuck on this last one.

Since $@ is globally transformed into argv in the parser, it'll still return set argv x $argv.

Tried adding more logic to parseSetExpr(), but unsure how to skip the $@ -> argv translation for specific lines.

If we skip it for lines akin to set -- x "$@", we can translate it to the correct fish output set -a argv x.