named: support PostgreSQL :: cast directly after a named param

[c-tonneslan]

Closes #983.

sqlx.Named was bailing with `unexpected ':' while reading named param at N` on Postgres queries like:

```sql
SELECT :boundary::jsonb FROM ...
```

because the colon parser only knew about `::` as a way to escape a single colon — it had no story for the cast operator glued directly to the trailing edge of a named parameter.

When `:` lands while we've already built up a name, the new code emits the bindvar for that name and then peeks at the next byte. If it's another `:`, the whole `::` is treated as a Postgres cast and passed through to the rebound query. Otherwise the trailing `:` is written as literal text.

The bindvar-emission switch is now factored into an `emitName` helper so the new path and the existing end-of-name path stay in sync.

The `'a::b::c'` escape behavior inside SQL string literals is intentionally left alone — that case is ambiguous between Postgres cast and escaped colon and isn't what #983 is about.

Test plan

```
go test ./...
```

Added a case to TestCompileQuery covering `:boundary::jsonb, :id::text` across all bind types.

[mrj0]

fyi I have a fix here https://github.com/Vinovest/sqlx/blob/main/bind.go#L191 using sqltokenizer that fixes this and other problems with the parser

[itxaiohanglover]

This is a clean approach — extracting emitName avoids the duplication and the test cases cover the main scenarios. The sqltokenizer route would be more invasive for what's ultimately a 2-character lookahead. Any chance this could get reviewed?