feat: intermediate type computer (#600)

Closes partially #541.

### Summary of Changes

Port almost the entire functionality of the Xtext type computer to
Langium. Only the following tasks are still missing for feature parity,
but they require work in other areas of the language first:

* Elvis operator with nullable left side (**needs type checker**)
* Parameters of block lambdas passed as arguments (**needs scoping**)
* Parameters of expression lambdas passed as arguments (**needs
scoping**)

src/language/formatting/safe-ds-formatter.ts

@@ -313,7 +313,7 @@ export class SafeDsFormatter extends AbstractFormatter {
         const formatter = this.getNodeFormatter(node);
 
         if (annotationCallsOrEmpty(node).length > 0) {
-            if (node.static) {
+            if (node.isStatic) {
                 formatter.keyword('static').prepend(newLine());
             } else {
                 formatter.keyword('attr').prepend(newLine());
@@ -421,7 +421,7 @@ export class SafeDsFormatter extends AbstractFormatter {
         const formatter = this.getNodeFormatter(node);
 
         if (annotationCallsOrEmpty(node).length > 0) {
-            if (node.static) {
+            if (node.isStatic) {
                 formatter.keyword('static').prepend(newLine());
             } else {
                 formatter.keyword('fun').prepend(newLine());
@@ -550,7 +550,7 @@ export class SafeDsFormatter extends AbstractFormatter {
         const formatter = this.getNodeFormatter(node);
 
         if (annotationCallsOrEmpty(node).length === 0) {
-            if (node.variadic) {
+            if (node.isVariadic) {
                 formatter.property('name').prepend(oneSpace());
             }
         } else {