[linter] Misleading runtime preload recommendation when using ESM

The linter currently doesn't use a separate recommendation for ESM preloads:

https://github.com/ampproject/amp-toolbox/blob/153d76b722906aec1cb4a6c65d88f0c4d418630d/packages/linter/src/rules/RuntimeIsPreloaded.ts#L13

This is misleading, and might even lead to frustration if users blindly apply the recommendation and it doesn't solve the issue.

The linter should properly differentiate between:
1. `.js` with `rel=preload`
2. `.mjs` with `rel=preload`
3. `.mjs` with `rel=modulepreload`

The recommendation should then be generated accordingly so that applying the recommendation verbatim will actually fix the shown issue.

Start at packages/linter/src/rules/RuntimeIsPreloaded.ts around line 13 and trace how the preload recommendation is generated. Verify the three listed .js/.mjs combinations, then confirm that each recommendation matches its preload type and fixes the reported issue when applied verbatim.

Written by the indexing model from the issue text.