fix: consume the -- separator in every addon argument source

[acinader]

Applies on top of plaintextaccounting#2699, against that branch.

plaintextaccounting#2699 applies breakAtFirstSeparator to the command line only, so a -- written
in a config file section or a command alias was not properly processed

[argvdump] section containing "alpha -- beta", run as: hledger --conf CONF argvdump gamma

main            argv: [alpha] [beta] [gamma]
#2699           argv: [alpha] [--] [beta] [gamma]
this PR         argv: [alpha] [beta] [gamma]

It applies to command aliases as well as [COMMAND] sections:

[alias] ad = argvdump -- --x, run as: hledger ad y

main            argv: [--x] [y]
#2699           argv: [--] [--x] [y]
this PR         argv: [--x] [y]

Per source

The sources are joined as
conf <> alias <> cli, so a -- from a config section or alias falls before
the command line args and moves them all into the "after" partition, where
hledger's own options are deliberately not stripped.

argv: [alpha] [beta] [--conf] [CONF] [gamma]

So each source consumes its own first --:

consumeSeparator as = let (bs, cs) = breakAtFirstSeparator as in dropCliSpecificOpts bs <> cs
addonargs = concatMap consumeSeparator
              [supportedgenargsfromconf, confcmdargs0, aliasargs, cliargswithoutcmd]

This also makes the documented -- -n escape hatch reachable from a config
section and from an alias, not only from the command line. Before it, there was
no way to get -n to an addon except by typing it on the command line.

Tests

Three new cases in addons.test cover a config file section, a command alias,
and the escape hatch. A fourth covers an argument containing an apostrophe:
case 13 ("Arguments needing shell quoting are preserved") uses a b and $x,
both of which are delivered literally on main too, so nothing in the suite
currently fails without plaintextaccounting#2699's argv change. An apostrophe is the input that
distinguishes them.

addons.test 17/17, full functional suite 1746/1746, builds warning-free
under -Werror.

Note these cases only run locally for now — all six CI workflows exclude
/addons (see plaintextaccounting#2710).

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