Add support for placeholders in queries (%s, %(foo)s, ? etc.)

Most Postgres connectors support placeholders in queries to be bound during execute time, such as

-- PEP-249: format
select id from products p where p.event_id = %s;

-- PEP-249: pyformat
select id from products p where p.event_id = %(event_id)s

-- PEP-249: qmark
select id from products p where p.event_id = ?

-- PEP-249: named
select id from products p where p.event_id = :event_id

-- PEP-249: numeric
select id from products p where p.event_id = :1

It would be useful if queries containing placeholders could be executed using the PGSQL: Execute PostgreSQL Query command with a way to supply parameters for placeholders, such as

1. Ask the user for values via interactive input,
2. Open a pane with a table of discovered placeholders and editable cells for values, or
3. Open an editor with discovered placeholders in eg. JSON or YAML syntax

Start with the PGSQL: Execute PostgreSQL Query command and inspect how it currently receives and executes query text. Compare the listed placeholder syntaxes and determine which parameter-entry approach should be implemented; done means a supported placeholder query can receive bound values and execute successfully.