feat: Implement query cancellation

[krlmlr]

Closes #514.

library(DBI)
con <- dbConnect(duckdb::duckdb())

dbExecute(con, "CREATE TABLE data AS SELECT unnest(generate_series(1, 100000)) AS a")

tryCatch(
  dbGetQuery(con, "SELECT COUNT(*) FROM data JOIN data AS data2 ON data.a != data2.a"),
  interrupt = function(e) {
    message("Interrupt handled in R")
  }
)

dbDisconnect(con)