Add support for two arg directive

[breathe]

Adds support for two arg version of pggen.arg()

pggen.arg('param', <default value expression>)

• default value expression can be arbitrary sql excepting that it must not contain any calls to pggen.arg(...)
• select pggen.arg('param', <default value expression>) compiles to select coalesce($1, <default value expression>)
• the code generator produces a nullable go type for param so that nil may be passed into the query
• technically this supports an arbitrary number of arguments -- eg pggen.arg('param', null::int, 1) would compile to coalesce($1, null::int, 1) -- which doesn't seem particularly useful but neither does it seem harmful enough to add logic to prevent

Closes https://github.com/jschaf/pggen/issues/86 and https://github.com/jschaf/pggen/issues/85

(sorry for the import reordering ... the vscode go extension does this by default -- not entirely clear to me why vscode go doesn't just comply with whatever the best/agreed upon default formatting conventions are for the ecosystem ... -- happy to update to remove those changes if you want)

[breathe]

I'm closing this due to lack of interest