Make fast-json-stringify optional

[hoangvvo]

Thanks for the great project.

Is your feature request related to a problem? Please describe.

I'm experimenting with using graphql-jit in the browser. Currently graphql-jit is bundled with fast-json-stringify, which makes its size extremely big to be used in browser.

https://bundlephobia.com/result?p=graphql-jit

Removing fast-json-stringify would cut the size in half.

Describe the solution you'd like

Currently, we have the option compilerOptions.customJSONSerializer, which accepts a boolean value. However the option does not provide much use because it only switches between JSON.stringify and fastJSON, not really 'custom'.

My proposal is that we replace it with a user-supplied function that accepts CompilerContext and returns the stringify function.

[mcollina]

That would be ok for me!