enhance: enable shell completions

[njhale]

Enable basic shell completions for bash, zsh, fish, and
powershell by exposing the completion command automatically
generated by Cobra. The command is hidden, so it doesn't show up
in the help text or completions.

$ source <(gptscript completion zsh)
$ gptscript --[tab]
 -- completions --
--assemble            -- Assemble tool to a single artifact, saved to --output ($GPTSCRIPT_ASSEMBLE)
--cache               -- Disable caching ($GPTSCRIPT_CACHE)
--cache-dir           -- Directory to store cache (default: $XDG_CACHE_HOME/gptscript) ($GPTSCRIPT_CACHE_DIR)
--debug               -- Enable debug logging ($GPTSCRIPT_DEBUG)
--dump-state          -- Dump the internal execution state to a file ($GPTSCRIPT_DUMP_STATE)
--help                -- help for gptscript
--input               -- Read input from a file ("-" for stdin) ($GPTSCRIPT_INPUT)
--list-models         -- List the models available and exit ($GPTSCRIPT_LIST_MODELS)
--list-tools          -- List built-in tools and exit ($GPTSCRIPT_LIST_TOOLS)
--listen-address      -- Server listen address ($GPTSCRIPT_LISTEN_ADDRESS)
--openai-api-key      -- OpenAI API KEY ($OPENAI_API_KEY)
--openai-api-type     -- OpenAI API Type (valid: OPEN_AI, AZURE, AZURE_AD) ($OPENAI_API_TYPE)
--openai-api-version  -- OpenAI API Version (for Azure) ($OPENAI_API_VERSION)
--openai-base-url     -- OpenAI base URL ($OPENAI_BASE_URL)
--openai-org-id       -- OpenAI organization ID ($OPENAI_ORG_ID)
--output              -- Save output to a file, or - for stdout ($GPTSCRIPT_OUTPUT)
--quiet               -- No output logging ($GPTSCRIPT_QUIET)
--server              -- Start server ($GPTSCRIPT_SERVER)
--sub-tool            -- Use tool of this name, not the first tool in file ($GPTSCRIPT_SUB_TOOL)
--version             -- version for gptscript

This implementation produces file path completions for ALL positional and flag args.
Support for customizing completions for individual arguments will
require new features in github.com/acorn-io/cmd and should be
addressed in a followup.

[njhale]

looks like this change is causing some issues, going to put it into draft hold off on merging until I fix it.

[njhale]

Okay, setting TraverseChildren = true did the trick.