deps: upgrade Babel preset-env, remove now redundant plugins

- optional-chaining and nullish-coalescing are now part of
  @babel/preset-env, so no need to explicitly install or configure
  them anymore
  - previously they were new syntax and TSDX adopted them as soon as
    they landed in TS
  - can view current compat table here:
    https://github.com/babel/babel/blob/e498bee10f0123bb208baa228ce6417542a2c3c4/packages/babel-compat-data/data/plugins.json#L24

package.json

@@ -44,13 +44,11 @@
     "@babel/core": "^7.4.4",
     "@babel/helper-module-imports": "^7.0.0",
     "@babel/plugin-proposal-class-properties": "^7.4.4",
-    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.7.4",
-    "@babel/plugin-proposal-optional-chaining": "^7.7.5",
     "@babel/plugin-syntax-dynamic-import": "^7.2.0",
     "@babel/plugin-transform-regenerator": "^7.4.5",
     "@babel/plugin-transform-runtime": "^7.6.0",
     "@babel/polyfill": "^7.4.4",
-    "@babel/preset-env": "^7.4.4",
+    "@babel/preset-env": "^7.11.0",
     "@rollup/plugin-commonjs": "^11.0.0",
     "@rollup/plugin-json": "^4.0.0",
     "@rollup/plugin-node-resolve": "^7.1.0",

src/babelPluginTsdx.ts

@@ -82,10 +82,6 @@ export const babelPluginTsdx = babelPlugin.custom(() => ({
           name: '@babel/plugin-proposal-class-properties',
           loose: true,
         },
-        // Adds syntax support for optional chaining (.?)
-        { name: '@babel/plugin-proposal-optional-chaining' },
-        // Adds syntax support for default value using ?? operator
-        { name: '@babel/plugin-proposal-nullish-coalescing-operator' },
         {
           name: '@babel/plugin-transform-regenerator',
           async: false,