✨ add flutter completions (#792)

Just a barebones `flutter` command and `flutter pub` commands, the most
used ones

custom-completions/flutter/flutter-completions.nu

@@ -0,0 +1,41 @@
+export extern "flutter" [
+    command?: string@"nu-complete flutter commands"
+    --help(-h)              # Print this usage information.
+    --verbose(-v)           # Noisy logging, including all shell commands executed.
+    --device-id(-d)         # Target device id or name (prefixes allowed).
+    --version               # Reports the version of this tool.
+    --enable-analytics      # Enable telemetry reporting each time a flutter or dart command runs.
+    --disable-analytics     # Disable telemetry reporting each time a flutter or dart command runs, until it is re-enabled.
+    --suppress-analytics    # Suppress analytics reporting for the current CLI invocation.
+]
+
+def "nu-complete flutter commands" [] {
+    ^flutter --help 
+    | into string 
+    | str replace --regex --multiline '(Manage[\s\S]*(?=Flutter SDK))' '' 
+    | lines 
+    | filter { str starts-with "  " } 
+    | each { str trim }
+    | parse "{value} {description}"
+    | str trim
+}
+
+export extern "flutter create" [
+    project: string         # project to create
+]
+
+export extern "flutter pub" [
+    command?: string@"nu-complete pub commands"
+]
+
+def "nu-complete pub commands" [] {
+    ^flutter pub --help 
+    | into string 
+    | str replace --regex --multiline '(Commands[\s\S]*(?=Available subcommands))' '' 
+    | lines 
+    | filter { str starts-with "  " } 
+    | each { str trim }
+    | parse "{value} {description}"
+    | str trim
+}
+