docs: replace .js with .mjs in links to files in ndoc comments (#989)

lib/index.mjs

@@ -124,12 +124,12 @@ function normalizeLinkText (url) {
  * MarkdownIt provides named presets as a convenience to quickly
  * enable/disable active syntax rules and options for common use cases.
  *
- * - ["commonmark"](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/commonmark.js) -
+ * - ["commonmark"](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/commonmark.mjs) -
  *   configures parser to strict [CommonMark](http://commonmark.org/) mode.
- * - [default](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/default.js) -
+ * - [default](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/default.mjs) -
  *   similar to GFM, used when no preset name given. Enables all available rules,
  *   but still without html, typographer & autolinker.
- * - ["zero"](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/zero.js) -
+ * - ["zero"](https://github.com/markdown-it/markdown-it/blob/master/lib/presets/zero.mjs) -
  *   all rules disabled. Useful to quickly setup your config via `.enable()`.
  *   For example, when you need only `bold` and `italic` markup and nothing else.
  *
@@ -146,7 +146,7 @@ function normalizeLinkText (url) {
  *   Can be useful for external highlighters.
  * - __linkify__ - `false`. Set `true` to autoconvert URL-like text to links.
  * - __typographer__  - `false`. Set `true` to enable [some language-neutral
- *   replacement](https://github.com/markdown-it/markdown-it/blob/master/lib/rules_core/replacements.js) +
+ *   replacement](https://github.com/markdown-it/markdown-it/blob/master/lib/rules_core/replacements.mjs) +
  *   quotes beautification (smartquotes).
  * - __quotes__ - `“”‘’`, String or Array. Double + single quotes replacement
  *   pairs, when typographer enabled and smartquotes on. For example, you can
@@ -272,15 +272,15 @@ function MarkdownIt (presetName, options) {
    * md.renderer.rules['my_token'] = myToken
    * ```
    *
-   * See [[Renderer]] docs and [source code](https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.js).
+   * See [[Renderer]] docs and [source code](https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.mjs).
    **/
   this.renderer = new Renderer()
 
   /**
    * MarkdownIt#linkify -> LinkifyIt
    *
    * [linkify-it](https://github.com/markdown-it/linkify-it) instance.
-   * Used by [linkify](https://github.com/markdown-it/markdown-it/blob/master/lib/rules_core/linkify.js)
+   * Used by [linkify](https://github.com/markdown-it/markdown-it/blob/master/lib/rules_core/linkify.mjs)
    * rule.
    **/
   this.linkify = new LinkifyIt()

lib/renderer.mjs

@@ -137,7 +137,7 @@ function Renderer () {
    * }
    * ```
    *
-   * See [source code](https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.js)
+   * See [source code](https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.mjs)
    * for more details and examples.
    **/
   this.rules = assign({}, default_rules)