Remove DirectIndexString from Base

[bkamins]

It was discussed in #23805 that DirectIndexString should be moved from Base to LegacyStrings.

I can start working on it. However, I am not sure about the expected details of this process. In particular:

• should it be moved to LegacyStrings first and after it is merged there be removed from Base (those are two separate repositories);
• nextind and prevind for DirectIndexString are inconsistent with other AbstractString types - should this be fixed or left as is for backward compatibility?
• I understand that this code should be included only under versions later than 0.6 - is the best practice for that to wrap the code in if VERSION >= v"0.6.0" block?

CC @nalimilan @ararslan @StefanKarpinski

[ararslan]

I recommend taking a look at the structure of LegacyStrings. As an example, I have a PR up there that adds RevString. Putting stuff in there is pretty straightforward. You just move the type and all of the Base methods for it.

[nalimilan]

No need to change the behavior of prevind and nextind since DirectIndexString will be semi-deprecated in LegacyStrings anyway.

[bkamins]

close?