[GH-8313] Allow use of custom HTML-Tags in PHP

[NReib]

• register CustomHtmlExtension to x-php5 mimeType
• UnknownAttribute hint used to be at wrong position in source when embedded in PHP

Example:
customs.json

{
	"elements": {
		"y-customdefined": {
			"attributes": {
				"exists": {}
			}
		}
	},
	"attributes": {"globaldefined": {}}
}

PHP script including custom tags, so far none of the tags defined in customs.json will be valid

<x-customnotdefined><?php echo 'test'; ?></x-customnotdefined>
<y-customdefined exists="" noexists=""><?php echo 'test'; ?></y-customdefined>
<y-customdefined exists=""></y-customdefined>
<y-customdefined globaldefined=""></y-customdefined>
<y-customdefined noexists=""><?php echo 'test'; ?></y-customdefined>

[matthiasblaesing]

@NReib thank you for looking into this. The change looks sane to me and looks simple enough to merge without new tests. I manually verified the location by checking the demo document and checking the reported positions. While it is not perfect after this change, it is an improvement.

I cleared the tests for running, added this to the NB26 milestone, @mbien already took care for adding the necessary labels to let the right unittest suites run.

[mbien]

restarting the tests, since approving the test run will unfortunately not take the new labels into account.

[matthiasblaesing]

darn thanks @mbien

[matthiasblaesing]

Looks sane to me. @NReib please indicate if it is ok to merge from your perspective.

[NReib]

@matthiasblaesing From what I saw so far it would be ok to merge. We will use this feature in the future at my work but probably after the next release. So maybe then I will gain more insights.