Add/Fix ability to use HTML fragments

[BenSouchet]

Issue #4811 Add/Fix ability to use HTML fragments

Description of changes:

I fixed the existing code handling the HTML fragment feature, I haven't remove the obsolete DocumentFragment class but this class either was never really finished or pieces were lost over time.

I decided to use the exact same route as for non-fragment document, and do the required work to reconnect with the existing features.

I test it by compiling with node ./Makefile.dryice.js on a clean new web server and it's works.

Risks

Since the feature didn't work for at least the last 4/5 years I don't think new bugs will be introduced with these changes.
Few have tried to use fragment (see issue #3518), but was face to the incomplete DocumentFragment class.

How to use Fragment

You need to pass to the HTML mode an object with a property called fragmentContext and the value need to be the name of a valid DOM Element tag name (head, body, div, footer, ...) ensure it in lowercase without extra space(s).

After a discussion with @andrewnester , my setup is:

let containerId = 'my-awesome-ace';

let editor = ace.edit(containerId);

let HtmlMode = require("ace/mode/html").Mode;
editor.session.setMode(new HtmlMode({fragmentContext: "body"});

If for a reason I ignore you have an error telling you require("ace/mode/html") return undefined

Do something like:

let config = require("ace/config");
let net = require("ace/lib/net");

function _initialize() {
    let containerId = 'my-awesome-ace';

    let editor = ace.edit(containerId);

    let HtmlMode = require("ace/mode/html").Mode;
    editor.session.setMode(new HtmlMode({fragmentContext: "body"});
}

net.loadScript(config.moduleUrl("ace/mode/html", "mode"), _initialize);

If you use the no-conflic version of ace, please prefix the require with ace..

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #4812 (cb513e0) into master (57ded09) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4812      +/-   ##
==========================================
+ Coverage   71.12%   71.16%   +0.04%     
==========================================
  Files         554      554              
  Lines       55711    55716       +5     
  Branches    10419    10420       +1     
==========================================
+ Hits        39623    39652      +29     
+ Misses      16088    16064      -24     

Flag	Coverage Δ
unittests	71.16% <ø> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/ace/mouse/default_gutter_handler.js	25.96% <0.00%> (-0.78%)	⬇️
lib/ace/tooltip.js	43.75% <0.00%> (+0.27%)	⬆️
lib/ace/editor.js	81.70% <0.00%> (+1.97%)	⬆️

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