Fix broken webpack build in browser

[dapplion]

Motivation

Lightclient UI build emits warnings when trying to import .d.ts files with a require statement.

Failed to compile.

./node_modules/@chainsafe/lodestar-config/lib/chainConfig/presets/mainnet.d.ts 2:7
Module parse failed: Unexpected token (2:7)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
| import { IChainConfig } from "../types";
> export declare const chainConfig: IChainConfig;
| //# sourceMappingURL=mainnet.d.ts.map

./node_modules/@chainsafe/lodestar-config/lib/chainConfig/presets/minimal.d.ts 2:7
Module parse failed: Unexpected token (2:7)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
| import { IChainConfig } from "../types";
> export declare const chainConfig: IChainConfig;
| //# sourceMappingURL=minimal.d.ts.map

https://github.com/ChainSafe/eth2-light-client-demo/runs/2914224290

Description

Change how the config presets are imported to match lodestar-params

lodestar/packages/params/src/activePreset.ts

Lines 11 to 20 in dc16c5a

	switch (process.env.LODESTAR_PRESET) {
	case PresetName.minimal:
	ACTIVE_PRESET = PresetName.minimal;
	preset = minimal;
	break;
	case PresetName.mainnet:
	default:
	ACTIVE_PRESET = PresetName.mainnet;
	preset = mainnet;
	break;

It does not cause the warnings anymore, and does not need using a dynamic require statement.

Note that the ACTIVE_PRESET env technically arbitrary so it's a bit fishy to have a dynamic non-controlled import there when it's not strictly necessary.

[codeclimate]

Code Climate has analyzed commit 755183b and detected 0 issues on this pull request.

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[dapplion]

Prevent running extra work that may not be needed

[wemeetagain]

Thanks :)