[linter] Misleading runtime preload recommendation when using ESM

[schlessera]

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

amp-toolbox/packages/linter/src/rules/RuntimeIsPreloaded.ts

Line 13 in 153d76b

: this.warn('<link href=https://cdn.ampproject.org/v0.js rel=preload> is missing');

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.

[schlessera]

The linter should recognize both 2. and 3. but only recommend one of the two, obviously. I think to remember that there was currently a transitional period that favored one over the other, so I'm unsure what the final verdict is.