move back ivas_dec() into ivas_dec.c

• Related issues: #880 (closed)
• Requested reviewers: @pihlajakuja or @norvell or @multrus

Reason why this change is needed

• Inconsistent function and file names for ivas_dec().
• The changes follow the proposal and discussion at !1520 (comment 106427).

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

• Allis BE.

Related to #880 (closed)