add capture-foreign-env module (#1099)

This is a modified version of [capture-foreign-env command from nushell
wiki](https://www.nushell.sh/cookbook/foreign_shell_scripts.html#detailed-explanation-of-capture-foreign-env).
Unlike the wiki version It relies on `null_byte` instead of `newline` as
a delimiter for environment variables dumped by the `env` command which
allows it to support multi-line foreign environment variables.

modules/capture-foreign-env/README.md

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+# capture-foreign-env
+
+This is a modified version of the [capture-foreign-env command from the nushell wiki](https://www.nushell.sh/cookbook/foreign_shell_scripts.html#detailed-explanation-of-capture-foreign-env).
+
+Unlike the wiki version It relies on `null_byte` instead of `newline` as a delimiter for environment variables dumped by the `env` command which allows it to support multi-line foreign environment variables.

modules/capture-foreign-env/mod.nu

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+# Returns a record of changed env variables after running a non-nushell script's contents (passed via stdin), e.g. a bash script you want to "source"
+export def main [
+  --shell (-s): string = /bin/sh
+  # The shell to run the script in
+  # (has to support '-c' argument and POSIX 'env', 'echo', 'eval' commands)
+  --arguments (-a): list<string> = []
+  # Additional command line arguments to pass to the foreign shell
+] {
+  let script_contents = $in;
+  let env_out = with-env { SCRIPT_TO_SOURCE: $script_contents } {
+    ^$shell ...$arguments -c `
+    env -0
+    echo -n '<ENV_CAPTURE_EVAL_FENCE>'
+    eval "$SCRIPT_TO_SOURCE"
+    echo -n '<ENV_CAPTURE_EVAL_FENCE>'
+    env -0 -u _ -u _AST_FEATURES -u SHLVL`
+  }
+  | split row '<ENV_CAPTURE_EVAL_FENCE>'
+  | {
+    before: ($in | first | str trim --char (char nul) | split row (char nul))
+    after: ($in | last | str trim --char (char nul) | split row (char nul))
+  }
+
+  $env_out.after
+  | where { |line| $line not-in $env_out.before }
+  | parse "{key}={value}"
+  | transpose --header-row --as-record
+}