How to add typewriter effect in Davinci Resolve or Fusion?
In the Text+ tool there is a control for write-on that does exactly what you want. To quote from the Fusion 9 Tool Reference:


  Write On 
  This range control is used to quickly apply simple Write On
  and Write Off effects to the text. To create a Write On effect,
  animate the End portion of the control from 1 to 0 over the length of
  time required. To create a Write Off effect, animate the Start portion
  of the range control from 0 to 1.


When they say Write-On, unfortunately they don't mean the hand writing effect more commonly called write-on (I got too excited when I first saw it), but rather what you called the "typewriter effect". Video tutorial here