Norg/Treesitter, or external providers

[hedyhli]

• Norg: Feature Request - Support Neorg simrat39/symbols-outline.nvim#190
• zk: symbols-outline integration with zk for backlinks zk-org/zk-nvim#134

There could be a providers.priority table that lists providers to try, in that order. These would be appended to form import path outline.providers.<value>.

Hence external plugins can have a lua/outline/providers/<plugin>.lua and users of those plugins can simply add "plugin" to the priority list and the external provider can be used.

todo

• standardize what functions each provider must have
• use the new config option and import them properly in providers/init.lua
• Norg support shouldn't be too hard. I'll create an example outline-norg plugin to this this eventually decided to add this into builtin providers since the module ended up being relatively lightweight. norg
• help files can include outline items of headings only (h[123456] nodes in treesitter). vimdoc (moved to vimdoc support #34)

[hedyhli]