Add custom completions for aws-cli (#1073)

Aws-cli ships with a dedicated completion binary that only needs to be
passed current commandline to work. Space is appended to returned
results so that completing subcommands can be done simply by pressing
tab multiple times

custom-completions/aws/aws-completions.nu

@@ -0,0 +1,13 @@
+def complete-commands [
+    context: string
+] {
+    with-env {COMP_LINE: $context} {
+        aws_completer
+        | lines
+        | each {|x| $"($x) "}
+    }
+}
+
+export extern "aws" [
+    ...command: string@complete-commands
+]

custom-completions/aws/readme.md

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+# Amazon Web Services CLI completions
+
+A Nushell extern definition and completers for [Amazon Web Services CLI `aws`](https://aws.amazon.com/cli/).
+
+This module utilizes the `aws_completer` binary that ships with `aws` to provide completions
+
+## How to use
+
+Add `source path/to/aws-completions.nu` to your `config.nu` (`$nu.config-path`).