move back ivas_dec() into ivas_dec.c

• Related issues: ivas-codec-pc/ivas-codec#880 (closed)
• Requested reviewers: @pihlajakuja or @norvell or @multrus

Reason why this change is needed

• Inconsistent function and file names for ivas_dec().

Description of the change

• Move principal IVAS decoder-related functions to ivas_dec.c
• Move principal IVAS internal-renderer-related functions to ivas_dec_render.c

Affected operating points

• All is BE.