feature request: a method to modify listenv using another list/listenv

Hi @HenrikBengtsson,

Possible enhancement in having a function to modify existing `listenv` with another `list`/ `listenv`, similar to how the function `utils::modifyList` works.

Here is an example of what it could be like:

UPDATED Code Chunk: ----
``` r
library(listenv)

modifyListenv <- function (x, val, keep.null = FALSE) {
stopifnot(
inherits(x, "listenv") || is.list(x),
inherits(val, "listenv") || is.list(val)
)
xnames <- names(x)
vnames <- names(val)[nzchar(names(val))]
if (keep.null) {
for (v in vnames) {
if (v %in% xnames
&& is.list(x[[v]])
&& (is.list(val[[v]]) || inherits(val[[v]], "listenv"))) {
x[v] <- list(modifyListenv(x[[v]], val[[v]], keep.null = keep.null))
} else if (v %in% xnames
&& inherits(x[[v]], "listenv")
&& (is.list(val[[v]]) || inherits(val[[v]], "listenv"))) {
modifyListenv(x[[v]], val[[v]], keep.null = keep.null)
} else {
x[v] <- list(val[[v]])
}
}
} else {
for (v in vnames) {
if (v %in% xnames
&& is.list(x[[v]])
&& (is.list(val[[v]]) || inherits(val[[v]], "listenv"))) {
x[[v]] <- modifyListenv(x[[v]], val[[v]], keep.null = keep.null)
} else if (v %in% xnames
&& inherits(x[[v]], "listenv")
&& (is.list(val[[v]]) || inherits(val[[v]], "listenv"))) {
modifyListenv(x[[v]], val[[v]], keep.null = keep.null)
} else {
x[[v]] <- val[[v]]
}
}
}
x
}

X = listenv(a=7, c="dummy")
Y = listenv(a=6,b=10)
Z = listenv(d = NULL)

modifyListenv(X,Y)
#> A 'listenv' vector with 3 elements ('a', 'c', 'b').
as.list(X)
#> $a
#> [1] 6
#>
#> $c
#> [1] "dummy"
#>
#> $b
#> [1] 10

modifyListenv(X,Z, keep.null = T)
#> A 'listenv' vector with 4 elements ('a', 'c', 'b', 'd').
as.list(X)
#> $a
#> [1] 6
#>
#> $c
#> [1] "dummy"
#>
#> $b
#> [1] 10
#>
#> $d
#> NULL

X = listenv(a=7, c="dummy")
Y = listenv(b=10, x = listenv(a=2))
Z = listenv(d = NULL, x= X)

modifyListenv(Z,Y)
#> A 'listenv' vector with 3 elements ('d', 'x', 'b').
as.list(Z$x)
#> $a
#> [1] 2
#>
#> $c
#> [1] "dummy"

X = listenv(a=7, c="dummy", d = list(e=1,f=2,g=3))
Y = listenv(b=10, d = list(f=20))
modifyListenv(X,Y)
#> A 'listenv' vector with 4 elements ('a', 'c', 'd', 'b').
as.list(X$d)
#> $e
#> [1] 1
#>
#> $f
#> [1] 20
#>
#> $g
#> [1] 3

X = listenv(a=7, c="dummy", d = list(e=1,f=2,g=listenv(h = 1, i = 2)))
Y = listenv(b=10, d = list(f=20, g=listenv(h = 10)))
modifyListenv(X,Y)
#> A 'listenv' vector with 4 elements ('a', 'c', 'd', 'b').
as.list(X$d$g)
#> $h
#> [1] 10
#>
#> $i
#> [1] 2
```

^{Created on 2022-05-27 by the [reprex package](https://reprex.tidyverse.org) (v2.0.1)}

Alittle bit messy at the moment :S But seems to do what is required :)