Creating my own file type with custom syntax highlighting

I want to add my own file type, .mcfunction complete with keywords and arguments to the keywords.

PhPStorm, lets me add filetypes and highlights words based on the keywords I give it, and it has 4 categories but no documentation how to use this window.

For example I want scoreboard  to be a keyword only if it's at the start of a line or if the word run is behind it. Eg:

scoreboard
execute as @s run scoreboard

However, one of my directories is named scoreboard so whenever I want to direct a function to that directory, phpstorm highlights the word scoreboard in the directory.

function namespace:setup/scoreboard/all

Also, I have no idea how todo arguments. For example objectives is only an argument for scoreboard so the word objectives should never be highlighted unless immediately behind it is the word scoreboard. Eg:

scoreboard objectives

The problem is, I can't find any documentation on this at all, how do I customize my file types keywords, or create a file type plugin to do what I want so all my .mcfunction files are syntax highlighted properly.