feat: add deprecated option to commands

[mcncl]

Description

This adds an option to commands for use of deprecated:"". It follows the same patterns as other options, like hidden:"" but accepts a message in the quotes to override the default (command "old-cmd" is deprecated).

Context

I've been using hidden:"" where I don't want users to find a command, however I also do want to steer them to the correct path if it just happens to be a command that they've memorised. I find it adds for a safer removal of deprecated commands, rather than having to constantly factor in backwards compat. For example, I've opted to add a better structure to my commands, such as; tool <thing> <action>, but still have things like tool <action> available which go against this pattern. I want folks to be able to see that it's deprecated and will be removed in the future.

Changes

• adds a new deprecated option to commands
  • adds to the required project files (kong.go, help.go and build.go)
  • adds testing

Notes

I did run gofumpt -l -w . before submitting this PR, as that's my formatter of choice, which has made some other changes due to its own rules, such as;

- type providerWithoutErrorCLI struct {
- }

+ type providerWithoutErrorCLI struct{}

If it's preferred that these are restored to their former glory then I can totally do that.

[alecthomas]

Yes please revert the formatter changes.

[mcncl]

@alecthomas done. I've reverted the formatting changes to both untouched files and files that I did amend that had existing choices for how things like var has been opted for over :=