feat: change default annotation convention on the fly

[WieeRd]

It is quite bothersome to modify Neogen config every time
when working on multiple projects with different doc annotation.
.generate() API does support convention option, but it's not ideal to call
:lua require("neogen").generate({ annotation_convention = "example" }) every time.

To solve this, I suggest 2 things:

Expose default convention table as part of API

There should be a table accessible through require("neogen").annotation_convention,
which contains the default value for annotation_convention option of .generate().

---@type table<string,string>
neogen.annotation_convention = {
	["{filetype}"] = "{convention}",
	["python"] = "numpydoc",
}

---buffer-local config; looked up before table above
---@type table<number,table<string,string>>
neogen.local_annotation_convention = {
	[bufnr] = {
		["{filetype}"] = "{convention}",
	},
	[3] = {
		["python"] = "numpydoc",
	},
}

Add a command to easily modify those tables

Something like :DocType[!] [filetype] <convention>.
When used with ! it sets buffer-local option.
When no filetype is given, it uses vim.bo.filetype.
(e.g. :DocType numpydoc or :DocType python numpydoc)

Adding features like these will make it easier to set project-local options
and try out different annotation conventions with minimal effort.