Check for unawaited promises with ESlint

[toinehartman]

Describe the bug

Our TS code base (or proposed changes) sometimes have unawaited promises.

Examples:

• Unify VFS interfaces #967 (comment)

• rascal-language-servers/rascal-vscode-extension/src/test/vscode-suite/dsl.test.ts

  Line 250 in 11e15f0

  ide.renameSymbol(editor, bench, "z");

• Fix parametric completion tests #1063

ESlint provides a rule to catch those: https://typescript-eslint.io/rules/no-floating-promises/
While at it, we should probably also check for misused promises: https://typescript-eslint.io/rules/no-misused-promises