Don't nuke workspace formatting

[Twey]

Currently any formatting inside workspace.members is destroyed. Notably this means that a Cargo.toml with a trailing comma in this member can never be correctly formatted with trailing multiline commas enabled:

$ cat tomlfmt.toml
multiline_trailing_comma = true

$ cat Cargo.toml 
[workspace]
members = [
    "arthur",
    "bar",
    "baz",
    "charles",
    "foo",
    "johnny",
    "quux",
]

$ cargo sort .
Checking ....
Finished: Cargo.toml for "." has been rewritten

$ cargo sort -c .
Checking ....
error: Dependencies for . are not sorted
error: Cargo.toml for . is not formatted

$ cargo sort .
Checking ....
Finished: Cargo.toml for "." has been rewritten

$ cargo sort -c .
Checking ....
error: Dependencies for . are not sorted
error: Cargo.toml for . is not formatted

This problem is exacerbated by the fact that the test sort::toml_edit_check, which would otherwise catch this issue, is negated: it checks that the workspace example is changed, which it is: the example

[workspace]

members = [
    "3",
    "4",
    "2",
    "1",
]

is modified to

[workspace]

members = [
    "3",
    "4",
    "2",
    "1"]

which is different. This is an unusual output format, but that's okay, because when running cargo sort for real, it will then immediately be reformatted back into

[workspace]

members = [
    "3",
    "4",
    "2",
    "1",
]

Additionally, this issue can only appear in projects in which a tomlfmt.toml is present. This is because, contrary to documentation, multiline_trailing_comma is actually false by default.

This PR:

• fixes the test (which then breaks)
• changes sort::sort_array to not throw away the trailing comma
• updates the default value of multiline_trailing_comma to true to match the documentation