Reuse generator when possible

[roni-frantchi]

Building a schema generator (more precisely, building the TypeScript program it uses), is very costly.
Trials show 5-6s in large projects.

Right now if JsonSchema.fromType<>() is used n times, this generator (and the TypeScript program that comes with it), will be built n times.

In most cases, the generator and its program can be reused - shaving off precious build minutes and memory pressure.
The only exception is when JsonSchema.fromType<>() is given the optional argument to change the generator config - in this case, a new generator will be built and used instead of the default one.

So in most cases, this PR will introduce an optimization making the transformer it n times faster.

Next optimization steps

Since we're running within a TypeScript program, maybe we'd would be able to reuse the program that is being transpiled right now instead of creating a new one.

That should shave off those final seconds.

const formatter = tjs.createFormatter(config);
const parser = tjs.createParser(program, config);
const generator = new tjs.SchemaGenerator(program, parser, formatter, config);
const schema = generator.createSchema(config.type);