Add preconditional invariant checking

[vinceau]

Description

This PR adds a consistent method for enforcing an invariant to be true prior to some method logic. We introduce two functions checkState which asserts any condition to be true, and checkExists which checks that it is not null or undefined. These functions provide type safety for the remainder of the code block.

Warning

These functions are not a replacement for error handling, rather it's to be used in cases where certain values or parameters must exist or be a certain value before executing the main logic.

Consider the following example:

 public async sendVerificationEmail() {
   const auth = getAuth();
   const user = auth.currentUser;
-  if (!user) {
-    throw new Error("User is not logged in.");
-  }
+  Preconditions.checkExists(user, "User is not logged in.");
 
   if (!user.emailVerified) {
     log.info(`Sending email verification`);
     await sendEmailVerification(user);
   }
 }

This is a common case where we want to execute certain logic (in this case, send an email), but before we do that we want to check that the user currently exists. This is a case where we can rewrite it with a Preconditions.checkExists(user) to make this easier to read.

Now consider this anti-example:

 async function fetchMediumNews(): Promise<NewsItem[]> {
   const response = await mediumJSONFeed("project-slippi");
-  if (response?.status !== 200) {
-    throw new Error("Error fetching Medium feed");
-  }
+  Preconditions.checkState(response?.status === 200, "Error fetching Medium feed");
 
   const result = response.response;
   // Do something with result
 }

It could be tempting to replace the conditional error with a Preconditions.checkState(response?.status === 200) however it's not really a condition that needs to be checked before the executing of core logic. This is simply error handling after we've fetched the data. Therefore we should not use a precondition check in this situation.