moves esbuild to an optional dependency; requires node 16 for frameworks

[bkendall]

Description

esbuild as a dependency causes issues with npm@6 since it will fail installs for optional dependencies where engines don't match (from npm-shrinkwrap). Moving esbuild to optionalDependencies allows node 14 installs (which default to npm@6) to continue working.

firebase-frameworks has a requirement of node >= 16.0.0 and npm will complain if its codepaths are used on lower versions of node. Enforcing a requirement of node 16 or higher in the awareness codepaths will prevent issues from arising there and should give us confidence that npm is >=8, where the optional dependencies issue above won't happen either (and therefore esbuild should have been installed with firebase-tools).

But, as a precaution, we must move and check the import for esbuild later in the runtime so that we don't try to load a dependency that may have failed to install (notably on node 14).

Scenarios Tested

I've been testing with a local nextjs app on nodes 14 and 16 with npms 6 and 8. It's behaving correctly so far as I can tell and installing the package with this change on node 14 does work again.

[buu700]

I've also been running into issues with npm 8.19.2 and node 16.16.0, which this may have already fixed (not 100% sure). To reproduce the issue, run npm install firebase-tools esbuild ; npm ci; it will fail with an error message about a bunch of missing esbuild packages with version 0.15.8.

As a temporary workaround, I created a version of the package with the shrinkwrap file removed (firebase-tools@github:buu700/firebase-tools-tmp).