feat: add defineConfig function in dts-cli/config

[ToppleTheNun]

add defineConfig function that can be used from dts.config.ts or
dts.config.js in order to provide IntelliSense for configuration
options.

move configuration loading logic into getDtsConfig and
getNormalizedDtsConfig in src/createBuildConfigs.ts.
getDtsConfig loads the appropriate dts.config.* file.
getNormalizedDtsConfig normalizes the loaded configuration by
providing default values for the otherwise nullable values in DtsConfig.

this setup would allow for easy use of babel-node if so desired
to allow the use of import statements in dts.config.js.

[aladdin-add]

the easiest way I can see is to just export:

export function defineConfig(opts: DtsOptions){
  return opts;
}

and the defineConfig should be exported from "dts-cli"(instead of "dts-cli/config"), to be the same as DtsOptions, thoughts?

[ToppleTheNun]

It should be available to be used as both

import { defineConfig } from "dts-cli/config";

and

import { defineConfig } from "dts-cli";

with the current implementation.

[aladdin-add]

but what's the benefit? Generally, it's no good to have multiple ways to do one thing.

[ToppleTheNun]

I originally went with both due to a desire to align with how vite/vitest do it, as well as not break users who are used to using just the import { DtsOptions, RollupOptions } from "dts-cli".

I'll change it to just the one import from the root.

[aladdin-add]

LGTM, thanks!