Tutorial for building VS code extension;
Using ANTLR parser
Writing an Editor for PlantUML - Strumenta
PlantUML is a tool to author UML diagrams using plain text. With it, we can describe diagrams of several kinds (including class diagrams, sequence diagrams, activity diagrams, and others) using a syntax that resembles pseudo-code, or what one would write in words and symbols on a piece of paper.