Expose is_array_insertion

[DavisVaughan]

Branched from #1, just look at the 2nd commit, I can rebase on main if we merge the first PR and then itll make more sense

unlink('test.json')

json_modify_file('test.json', c("foo", "bar"), 1:3)
cat(readLines("test.json"), sep = "\n")
#> {
#>     "foo": {
#>         "bar": [
#>             1,
#>             2,
#>             3
#>         ]
#>     }
#> }

# modifies at position
json_modify_file('test.json', list("foo", "bar", 1), 9999)
cat(readLines("test.json"), sep = "\n")
#> {
#>     "foo": {
#>         "bar": [
#>             1,
#>             9999,
#>             3
#>         ]
#>     }
#> }

# inserts at position
json_modify_file(
  'test.json',
  list("foo", "bar", 1),
  9998,
  is_array_insertion = TRUE
)
cat(readLines("test.json"), sep = "\n")
#> {
#>     "foo": {
#>         "bar": [
#>             1,
#>             9998,
#>             9999,
#>             3
#>         ]
#>     }
#> }

# inserts at end!
json_modify_file(
  'test.json',
  list("foo", "bar", -1),
  0,
  is_array_insertion = TRUE
)
cat(readLines("test.json"), sep = "\n")
#> {
#>     "foo": {
#>         "bar": [
#>             1,
#>             9998,
#>             9999,
#>             3,
#>             0
#>         ]
#>     }
#> }

unlink('test.json')