How can I convert "left clicked and moving mouse" event into "swipe" event?
Conceptually this is fine, but literally there's no point because a "swipe" is meaningless to the desktop interface. This is why a touchscreen driver emulates a mouse. It does the reverse of what you want, that is, it translates a swipe, which is meaningless to the GUI, into a set of mouse events.
That is different than a touchscreen phone, where there is no mouse and so mouse events, if you could attach one (perhaps some tablets allow this via bluetooth), would have to be translated into touchscreen events.
Most desktop browsers (such as those used in Raspbian, which is a desktop PC OS, not a mobile phone OS) can be scrolled with a mouse wheel if the cursor is in the main window. I believe this is another form of translation by the GUI stack; it tells the browser that the scroll bar on the side of the window is being moved. In any case, that's what you want to simulate.
There is a simple way to do that for mice that do not have a wheel; there's an example here, see also man evdev. Note that it means dedicating one of the buttons to this purpose, so when you hold it down, movement is treated as a scroll. This may be a bit of a problem for you if the touchscreen driver only simulates a single button mouse and/or won't hold a button down when you swipe.
However, there's a chance it could be made to work, and it is easy enough to try. Add a file to /etc/X11/xorg.conf.d/ as in the example (you may have to create that directory) and try tweaking it various ways. For the changes to take effect, you will have to shut down and restart the X server. /var/log/Xorg.0.log may then say something about whether your configuration was accepted or not.
A sure fire way to do this would be to write your own touchscreen driver, but that would be a considerable amount of work and require proficiency with C and getting familiar with the linux kernel API.
