Extend analysis to to custom file extensions; folding for non-Dart files

[simolus3]

To better support analyzer plugins on custom file endings, I've made two changes:

1. Register providers that aren't specific to Dart (all those we register on MODE_HTML) for extensions declared in additionalAnalyzerFileExtensions if the option to use the Angular plugin is turned on.
2. Use the analysis server folding for both html and custom files if enabled.

With a plugin I'm writing, I could verify that auto-complete, folding, outline, problems, fixes and navigation are all working with this setup.

Fixes #1981

Potential problems

In the linked issue, you asked whether the analysis server would send problematic responses when used with unsupported files. I played around with this as the plugin was turned off, and I didn't run into any problems.
Further, that shouldn't have any impact on the first change because it only effects users who explicitly added a file to additionalAnalyzerFileExtensions, which only makes sense when an appropriate plugin is available.

[DanTup]

Thanks! This doesn't seem to include any changes to util.isAnalyzable though, so it seems like it would not transmit your code changes to the server?

[DanTup]

Ugh, scratch that - it's already handled via additionalAnalyzerFileExtensions 😄

Thanks, I'll merge this for the next release!

[DanTup]

When making some changes, I noticed here we only add from additionalAnalyzerFileExtensions if analyzeAnglularTemplates is enabled. This seems incorrect and I can't find any explanation, so I'm moving it out of that check (so disabling AnalyzeAngularTemplates would not also remove custom extensions).

If you can think of a reason why it was done that way and/or why this may be a bad idea, let me know!

[simolus3]

I think I was afraid to unconditionally enable them so that analyzeAngularTemplates is the flag that enables analyzer plugins. I don't think it will cause any harm to make them independent.

Either way, thanks for informing me! I'll update the docs of my analyzer plugin to reflect this.

[DanTup]

Yeah I was thinking of renaming it, but it'd break people - but also, the flag is enabled by default so the risk seems low now :-)