Prepare for 1.0.0 Release

[datho7561]

It would be nice to do a 1.0.0 release of lemminx, given that lemminx has matured as a project. I feel like most of the important features have either been implemented, or there are technical reasons as to why we don't have them (XSD 1.1, Schematron).

In order to do the 1.0.0 release:

• We will need to do a graduation review (see https://www.eclipse.org/projects/handbook/#release-graduation)
• We will need to decide which parts of lemminx should be publicly accessible API. This will affect lemminx extensions

Also, please let us know if you think lemminx is missing any important features.

[angelozerr]

Before creating 1.0.0 we need to clean participant API. I mean:

• add cancelchecker in hover, in another participant?
• move participant in a proper package.
• use a request API for any participant

[datho7561]

We should also remove the legacy formatter (the new formatter is much better, I don't think there is a reason to use the legacy formatter): redhat-developer/vscode-xml#868 (comment)

[angelozerr]

For 1.0.0 I would like to manage all references with the currect xml.references support and remove all hard code which manage references for xsd, dtd. For instance for XSD, we could remove almost participant from https://github.com/eclipse/lemminx/tree/main/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/xsd/participants

The https://github.com/eclipse/lemminx/blob/main/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/xsd/participants/XSDDocumentLinkParticipant.java will able to remove when #1464 will be done (which I think it should be nice to implement for 1.0.0)

[angelozerr]

@mickaelistria @vrubezhny @kathrynkodama @cherylking @Treehopper please follow this issue (which should freeze the participant API I hope since we will create an 1.0.0 relase) because we will do some refactoring on some participant API (ex : move participant to the proper package like hover, add cancel checker for any API, etc).

Don't hesitate to provide your suggestion to provide the best patrticpant API .