Add zig and zvm completions (#1224)

Hey, I've been using this script for a while. 

Aside from adding documentation for the commands manually copied from
the help, it has custom completions for listing `zvm` versions, and
listing `zig build` targets.

As an unexpected nice thing, when completing for the `zig build`
command, it will rebuild the zig build script automatically, which is a
nice _side effect_.

---------

Co-authored-by: Gabriel Schneider <gbrls@0x4200.cafe>

Author: gbrls

custom-completions/zig/readme.md

@@ -0,0 +1,12 @@
+# `zig` custom completions
+
+This script provides custom completions for `zig` and `zvm`.
+It can be used by importing its exported commands via:
+
+```
+use path/to/zig/zig-completions.nu *
+```
+
+With `path/to/` being either the relative path of the file to your current working directory or its absolute path.
+
+The completion for `zig build` has a side-effect of rebuilding the build script to list the configured build steps.

custom-completions/zig/zig-completions.nu

@@ -0,0 +1,103 @@
+# zvm
+def "nu-complete zvm versions" [] {
+  ^zvm list
+  | lines
+}
+
+# switch between versions of Zig
+export extern "zvm use" [
+  --sync  #sync your current version of Zig with the repository
+  --help(-h)
+  version: string@"nu-complete zvm versions"
+]
+
+ # download and install a version of Zig
+export extern "zvm install" []
+# run a command with the given Zig version
+export extern "zvm run" []
+# list installed Zig versions. Flag `--all` to see remote options
+export extern "zvm list" []
+# remove an installed version of Zig
+export extern "zvm uninstall" []
+# remove build artifacts (good if you're a scrub)
+export extern "zvm clean" []
+# self-upgrade ZVM
+export extern "zvm upgrade" []
+# set ZVM's mirror list URL for custom Zig distribution servers, or set to "disabled" to download directly from ziglang.org
+export extern "zvm mirrorlist" []
+# set ZVM's version map URL for custom Zig distribution servers
+export extern "zvm vmu" []
+# Shows a list of commands or help for one command
+export extern "zvm help" []
+# =======
+
+# zig
+# =======
+
+def "nu-complete zig build" [] {
+  ^zig build -l
+  | lines
+  | parse --regex "^  ([a-z]+)" 
+  | get "capture0"
+}
+
+# Build project from build.zig
+export extern "zig build" [
+  step: string@"nu-complete zig build"
+]
+
+
+# Copy a package into global cache and print its hash
+export extern "zig fetch" []
+# Initialize a Zig package in the current directory
+export extern "zig init" []
+# Create executable from source or object files
+export extern "zig build-exe" []
+# Create library from source or object files
+export extern "zig build-lib" []
+# Create object from source or object files
+export extern "zig build-obj" []
+# Perform unit testing
+export extern "zig test" []
+# Create object for unit testing
+export extern "zig test-obj" []
+# Create executable and run immediately
+export extern "zig run" []
+# Look for simple compile errors in any set of files
+export extern "zig ast-check" []
+# Reformat Zig source into canonical form
+export extern "zig fmt" []
+# Minimize a bug report
+export extern "zig reduce" []
+# Convert C code to Zig code
+export extern "zig translate-c" []
+# Use Zig as a drop-in archiver
+export extern "zig ar" []
+# Use Zig as a drop-in C compiler
+export extern "zig cc" []
+# Use Zig as a drop-in C++ compiler
+export extern "zig c++" []
+# Use Zig as a drop-in dlltool.exe
+export extern "zig dlltool" []
+# Use Zig as a drop-in lib.exe
+export extern "zig lib" []
+# Use Zig as a drop-in ranlib
+export extern "zig ranlib" []
+# Use Zig as a drop-in objcopy
+export extern "zig objcopy" []
+# Use Zig as a drop-in rc.exe
+export extern "zig rc" []
+# Print lib path, std path, cache directory, and version
+export extern "zig env" []
+# Print this help and exit
+export extern "zig help" []
+# View standard library documentation in a browser
+export extern "zig std" []
+# Display native libc paths file or validate one
+export extern "zig libc" []
+# List available compilation targets
+export extern "zig targets" []
+# Print version number and exit
+export extern "zig version" []
+# Print Zen of Zig and exit
+export extern "zig zen" []