fs: fix setting stringArray config values from environment variables

After the config re-organisation, the setting of stringArray config
values (eg `--exclude` set with `RCLONE_EXCLUDE`) was broken and gave
a message like this for `RCLONE_EXCLUDE=*.jpg`:

    Failed to load "filter" default values: failed to initialise "filter" options:
    couldn't parse config item "exclude" = "*.jpg" as []string: parsing "*.jpg" as []string failed:
    invalid character '/' looking for beginning of value

This was caused by the parser trying to parse the input string as a
JSON value.

When the config was re-organised it was thought that the internal
representation of stringArray values was not important as it was never
visible externally, however this turned out not to be true.

A defined representation was chosen - a comma separated string and
this was documented and tests were introduced in this patch.

This potentially introduces a very small backwards incompatibility. In
rclone v1.67.0

    RCLONE_EXCLUDE=a,b

Would be interpreted as

    --exclude "a,b"

Whereas this new code will interpret it as

    --exclude "a" --exclude "b"

The benefit of being able to set multiple values with an environment
variable was deemed to outweigh the very small backwards compatibility
risk.

If a value with a `,` is needed, then use CSV escaping, eg

    RCLONE_EXCLUDE="a,b"

(Note this needs to have the quotes in so at the unix shell that would be

    RCLONE_EXCLUDE='"a,b"'

Fixes #8063

fs/config/configstruct/configstruct_test.go

@@ -204,9 +204,11 @@ func TestStringToInterface(t *testing.T) {
 		{"1m1s", fs.Duration(0), fs.Duration(61 * time.Second), ""},
 		{"1potato", fs.Duration(0), nil, `parsing "1potato" as fs.Duration failed: parsing time "1potato" as "2006-01-02": cannot parse "1potato" as "2006"`},
 		{``, []string{}, []string{}, ""},
-		{`[]`, []string(nil), []string{}, ""},
-		{`["hello"]`, []string{}, []string{"hello"}, ""},
-		{`["hello","world!"]`, []string(nil), []string{"hello", "world!"}, ""},
+		{`""`, []string(nil), []string{""}, ""},
+		{`hello`, []string{}, []string{"hello"}, ""},
+		{`"hello"`, []string{}, []string{"hello"}, ""},
+		{`hello,world!`, []string(nil), []string{"hello", "world!"}, ""},
+		{`"hello","world!"`, []string(nil), []string{"hello", "world!"}, ""},
 		{"1s", time.Duration(0), time.Second, ""},
 		{"1m1s", time.Duration(0), 61 * time.Second, ""},
 		{"1potato", time.Duration(0), nil, `parsing "1potato" as time.Duration failed: time: unknown unit "potato" in duration "1potato"`},