feat: add "on_save" option

[adoyle-h]

User can disable formatting on save.

[lukas-reineke]

In theory, I like this 👍

But there is a problem with this implementation. If there are more than 1 LSP attached to the buffer, they will overwrite each other.
If there is one LSP with on_save = false, and one with on_save = true, the autocommand will still be created. And on save, even the first LSP with on_save = false will format the buffer.
We need to check if on_save is true for each client that is attached to the buffer.

[adoyle-h]

If there are more than 1 LSP attached to the buffer, they will overwrite each other. If there is one LSP with on_save = false, and one with on_save = true, the autocommand will still be created. And on save, even the first LSP with on_save = false will format the buffer. We need to check if on_save is true for each client that is attached to the buffer.

This won't happen. Because format_options is relative to buffer filetype, not LSP client. The format_options are same for same filetype, even on_attach invoked with different client.

local format_options = vim.tbl_deep_extend('keep', M.format_options[vim.bo.filetype] or {}, { on_save = true })

[lukas-reineke]

This won't happen. Because format_options is relative to buffer filetype, not LSP client

Right, sorry. It's been a while since I worked on this. Ignore what I said

[lukas-reineke]

Overwriting from a different LSP is not possible 👍 , but this will still not properly work if the filetype changes.
You need to run the clear autocommands even if on_save is false

[adoyle-h]

If filetype changes, the on_attach function will not be triggered. And the previous LSP clients are still attaching on the buffer. You can try it by :set ft=<other filetype> after opened a file. I think this issue won't happen too.

[lukas-reineke]

It will if you use nvim-lspconfig, which most people do. I'd like to support this.