(refactor): make preset-env variable naming more explicit (jaredpalme…

…r#643)

- I was reading this and was confused which "preset" and "idx" it was
  talking about (despite having worked on some of the code here
  myself), so just make it more explicit

src/babelPluginTsdx.ts

@@ -102,27 +102,27 @@ export const babelPluginTsdx = babelPlugin.custom(() => ({
     const babelOptions = config.options || {};
     babelOptions.presets = babelOptions.presets || [];
 
-    const envIdx = babelOptions.presets.findIndex((preset: any) =>
+    const presetEnvIdx = babelOptions.presets.findIndex((preset: any) =>
       preset.file.request.includes('@babel/preset-env')
     );
 
     // if they use preset-env, merge their options with ours
-    if (envIdx !== -1) {
-      const preset = babelOptions.presets[envIdx];
-      babelOptions.presets[envIdx] = createConfigItem(
+    if (presetEnvIdx !== -1) {
+      const presetEnv = babelOptions.presets[presetEnvIdx];
+      babelOptions.presets[presetEnvIdx] = createConfigItem(
         [
-          preset.file.resolved,
+          presetEnv.file.resolved,
           merge(
             {
               loose: true,
               targets: customOptions.targets,
             },
-            preset.options,
+            presetEnv.options,
             {
               modules: false,
               exclude: merge(
                 ['transform-async-to-generator', 'transform-regenerator'],
-                (preset.options && preset.options.exclude) || []
+                (presetEnv.options && presetEnv.options.exclude) || []
               ),
             }
           ),