Add television custom-completions (#1239)

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

Author: gbrls

custom-completions/television/readme.md

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+# `television` custom completions
+
+This script provides custom completions for `television`.
+It can be used by importing its exported commands via:
+
+```
+use path/to/television/television-completions.nu *
+```
+
+With `path/to/` being either the relative path of the file to your current working directory or its absolute path.
+

custom-completions/television/television-completions.nu

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+def "nu-complete tv channels" [] {
+  ^tv list-channels
+  | lines
+}
+
+export extern "tv" [
+  channel: string@"nu-complete tv channels"
+]
+
+# Lists the available channels
+export extern "tv list-channels" []
+# Initializes shell completion ("tv init zsh")
+export extern "tv init" []
+# Generates standard shell tab-completion scripts for tv's various subcommands
+export extern "tv completions" []
+# Downloads the latest collection of channel prototypes from github and saves them to the local configuration directory
+export extern "tv update-channels" []
+# Print this message or the help of the given subcommand(s)
+export extern "tv help" []