Centralize template string functionality about langium

[danieldietrich]

I stumbled upon one issue: I need to find strings within strings in order to nested highlight whitespace.

Here is an Xtend example:

Langium example (whitespace = XXX + javascript code other than string contents):

const generated = s```
XXXclass A {
XXXXXX${members.map(member => {
XXXXXXXXXswtichX(member.type)X{
XXXXXXXXXXXXcaseX'method':XreturnXs```
XXXXXXXXXXXXXXXTODO: generate method
XXXXXXXXXXXX```
XXXXXXXXX}
XXXXXX}
XXX}

I do not have access to the AST (because it lives in the language server and the whitespace highlighter extension only has access to the editor contents), so I plan to implement a 'stack' parser that is aware of

1. comments (in order to skipt strings within comments)
2. strings (in order to find whitespace and align contents)
3. template literal substitutions (in order to recurse in javascript code from within a string context)

[spoenemann]

The changes in #825 break the highlighting provided by our vscode extension. @danieldietrich how shall we proceed with this? Remove the functionality for now, or make it work with the new API?

[danieldietrich]

@spoenemann could you please elaborate on the highlighting error by providing a screenshot? Let's see if there is a quick workaround...

[spoenemann]

It just doesn't show any whitespace highlighting. I assume it's because the s function has been replaced by two functions expandToString and expandToNode?