feat: extend builtin diagnostic to allow displayed text customization

[dansdantas]

This extends builtin.diagnostic opts, allowing to use a custom function. Solves #2544

[jamestrew]

How does this resolve #2544?

[dansdantas]

When writing a diagnostic picker, allow to pass a function in the text option that formats how text is displayed on the picker. First argument for that function is the diagnostic, allowing to add source into the displayed text. Then you can filter like normally you would do with telescope.

require('telescope.builtin').diagnostics({
    text = function(diagnostic)
        return vim.trim(diagnostic.message:gsub("[\n]", "") .. " (" .. diagnostic.source .. ")")
    end,
})

[jamestrew]

I think I would prefer we just implement that as oppose to adding a mechanism that allows people the option to implement it.