Simplify syntax for selecting fields with raw query

[IlyaSemenov]

Currently, selecting a custom field with raw sql looks like this:

r.table(
      db.post,
      {
        fields: {
          preview: (q) =>
            q.select({ preview: q.sql<string>`substr(text,1,100)` }),

This is too wordy, and, it breaks when a field is requested with an alias:

posts {
  my_preview: preview  # will not work
}

The syntax should rather be:

r.table(
      db.post,
      {
        fields: {
          preview: (q) => q.sql<string>`substr(text,1,100)`

with orchid-graphql wrapping the field select.