allow automatically exporting the global variables used in the function

[Jiefei-Wang]

This pull request supports auto export variables/packages from the manager to the worker when FUN in bplapply requires it.

Use case 1

x <- "hello"
bplapply(1, function(i) x, BPPARAM = SnowParam(2))
# [[1]]
# [1] "hello"

Use case 2

library(S4Vectors)
bplapply(1, function(i) DataFrame(), BPPARAM = SnowParam(2))
# [[1]]
# DataFrame with 0 rows and 0 columns

[Jiefei-Wang]

I did a minor cleanup to remove the empty lines.