VS Code's syntax highlighting is written with the TextMate grammar. It's a grammar that makes heavy use of regular expressions. So make sure you brush up your skill on backward/forward references and lookarounds, because all these can be used. TextMate is a greedy grammar, which means that unlike normal regular expressions it will not backtrack. Once a match is found, that will be used. The other important thing is that the grammar can be nested, but children of a match can exceed the parent match and therefore push the boundaries of the parent match.
A good document to read in more detail about TextMate: https://www.apeth.com/nonblog/stories/textmatebundle.html
The grammar for pug and markdown is written in JSON. You can edit that directly. The main grammar is written in yaml because that's easier to structure than JSON. However, VS Code expects a JSON format. Therefore you need to run the build:grammar` script afterward. This will produce a JSON version of the file. Afterward, you need to restart your extension window for the changes to take effect.
The snapshot test is run by vscode-textmate-test. The test command calls a node script to build the argument and execute the vscode-textmate-test command. If you updated the svelte grammar file, Be sure to add the test cases to the test/sample directory.
Then run the test to check if the update affects existing cases
pnpm run testUpdate existing snapshots to match the changes
pnpm run test -- --updateSnapshot