Sorry, but I can't push your suggestion.
Cause several problem (see my printscreen)

Other consequence: keyword like using is in a wrong color...

Currently the rules are:
po_, pi_, pio_ are detected as parameter variable
l_ are detected as local variable

Perhaps its not the rules you use. I don't know if we can improve this in a standard way.
You can also customize the colorization syntax for your usage see here

The same argument applies to your variables:

The point is that if we want to set rules, we can assume that the input parameters can start with the letter p***_.

Forcing the rule to only pi, po and pio parameters seems a little restrictive.
Above all considering the fact that it is not a fixed rule of plsql language, but a convention.
Also in the PR that I provided you there is the rule to highlight all the parameters followed by IN, OUT and IN OUT, which does not create any problems and is very similar to how the typescript highlights its parameters:

I have update the PR so you don't have problems with in out:

You can also customize the colorization syntax for your usage see here

If I change tmLanguage file it is overwritten when I update the extension.

here's my VSCode extension to help with authoring TextMate grammars https://marketplace.visualstudio.com/items?itemName=RedCMD.tmlanguage-syntax-highlighter