[html][css] [custom data] Reload custom data on user setting change or JSON file changes

[aeschli]

• listen to file changes on the configuration files and set a cutomdatachange event to the css/html language services

• listen to extension changes

• show information in the HTML/CSS language server logs which files have been read

[aeschli]

We need to decide what happens with the experimental setting, but if we go with it:

• listen to configuration changes (settings)
• listen to changes to the referenced files
• validate that the paths exists
• remove the restriction that it has to be a workspace setting

[octref]

I agree with all the rest, except "remove the restriction that it has to be a workspace setting", see #79911 (comment).