plpgsql: implement ALIAS FOR variable declarations

**Is your feature request related to a problem? Please describe.**

PostgreSQL's PL/pgSQL allows declaring an alias for a function
parameter or another variable using `ALIAS FOR`:

```sql
CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
DECLARE
amount ALIAS FOR subtotal; -- alternate name for the parameter
BEGIN
RETURN amount * 0.06;
END;
$$ LANGUAGE plpgsql;
```

CockroachDB rejects this at parse time:

- [`pkg/sql/plpgsql/parser/plpgsql.y:476-479`](https://github.com/cockroachdb/cockroach/blob/master/pkg/sql/plpgsql/parser/plpgsql.y#L476-L479)

```go
| decl_varname ALIAS FOR decl_aliasitem ';'
{
return unimplemented(plpgsqllex, "alias for")
}
```

The error is reported under the bare telemetry key
`unimplemented.alias for`, with no link to a tracking issue.

**Describe the solution you'd like**

Implement ` ALIAS FOR ` in the PL/pgSQL declaration
section, where `` may be either a function parameter
(by name or by `$N` positional reference) or another previously-declared
variable. References to the alias should resolve to the underlying
slot, so reads and assignments behave identically to using the original
name. Migrate the parser site to an issue-linked unimplemented helper
referencing this issue once the work lands.

Related: #169557 (split catch-all PL/pgSQL telemetry).

Jira issue: CRDB-63537

Epic CRDB-49018