✨ add flutter completions (nushell#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
+}
+