How could I make custom highlighting/error location for plugins?

[Frityet]

I created this plugin

---@class Diff
---@field start  integer The number of bytes at the beginning of the replacement
---@field finish integer The number of bytes at the end of the replacement
---@field text   string  What to replace

---@param  uri  string The uri of file
---@param  text string The content of file
---@return Diff[]?
function OnSetText(uri, text)
    local diffs = {}

    local function process_string(fmt, start_idx)
        local str = fmt:gsub("%b{}", function(curly)
            local inner = curly:sub(2, -2)
            return "\".."..inner.."..\""
        end)

        str = str:sub(3, -2)

        if str ~= fmt then
            return {
                start = start_idx,
                finish = start_idx + #fmt - 1,
                text = "\""..str.."\""
            }
        end
    end

    for start_idx, quote, inner in text:gmatch("()f(['\"])(.-)%2()") do
        if text:sub(start_idx - 2, start_idx - 1) == "--" then
            goto continue
        end

        local fmt = "f"..quote..inner..quote
        local diff = process_string(fmt, start_idx)
        if diff then
            table.insert(diffs, diff)
        end

        ::continue::
    end

    return diffs
end

and it works

fmt.mp4

but unfortunately the errors and highlighting doesnt properly, and the x: y: thingy at the end is kinda annoying

Anyone got a clue?