maint: add binary size checking to CI

[coilysiren]

What problem does this solve?

This fixes the problem of the binary size increasing over time, which is a non-starter for some users (like @oleorhagen!)

Solution description

A script that does the following psuedo-code:

go build urfave/cli --output ./urfave-cli

binarySize = checkBinarySize ( "./urfave-cli" )
expectedBinarySize = 5000

if binarySize > expectedBinarySize then
    fail the build, reason: binary too large
if binarySize < expectedBinarySize * 0.70% then
    fail the build, reason: binary is much smaller now ✨\
        please decrease the expected size

Potential Future Work

If we make the CLI modular (mentioned here #833 (comment)), we can assert different expected binary sizes for the "minimal" vs "complete" versions of the package.

Context

This idea was inspired by #1055