RFR: JDK-8275788: Create code element with suitable attributes for code snippets [v2]

[Hannes Wallnöfer]

> Please review a change to add a nested `<code>` element for snippet tags as well as HTML attributes for the snippet's `id` and `lang` attributes. The change is quite simple, I did however encounter some gotchas. Below is a short list of notes to explain some aspects of the change.
> 
>  - Both `id` and `lang` are only meaningful if they have non-empty values, so defining them with empty or missing values will cause the respective HTML attributes to be not defined.
>  - I decided to put the logic for extracting the `id` and `lang` attributes into the already quite long `SnippetTaglet.getInlineTagOutput` method as this is where the required preparation and infrastructure are located. Both values are then passed as additional arguments to `TagletWriter.snippetTagOutput` which is not great. I think it's ok for now, but maybe if we need to pass more values along that way we should reconsider this design.
>  - I'm not sure whether we should assume `lang=java` for internal snippets. This change currently does not, it only derives implicit `lang` attributes for external snippets based on the extension of the snippet source file.
>  - In `SnippetTaglet.languageFromFilename()` I tried to cover what I reckoned would be the most useful languages for javadoc users, including a few major JVM languages. I'm sure I forgot some, but I hope I got at least those file extensions right.
>  - We previously added a newline character to the snippet's `<pre>` element. I think this was done in order to make sure the element is rendered even if the snippet content is empty. This was fine with just the `<pre>` element, but with a nested `<code>` element that newline is visible in the browser. I therefore replaced the newline with `HtmlTree.EMPTY`. 
>  - For the test, I was able to convert the existing `testIdAndLangAttributes` test to cover explicit `id` and `lang` attributes. I added a new test called `testExternalImplicitAttributes` to test derived `lang` attributes.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8275786: Avoid use of String.toLowerCase() without locale

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6165/files
  - new: https://git.openjdk.java.net/jdk/pull/6165/files/b97ea5b7..1b418f54

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6165&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6165&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6165.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6165/head:pull/6165

PR: https://git.openjdk.java.net/jdk/pull/6165