feat(header): rename to heading

[lubber-de]

Description

As discussed in #2407 this PR renames header to heading.

• rename all .ui.header selectors to .ui.heading
• rename all @header... LESS variables to @heading...
• rename all related files for building to heading as well (so header.less and header.variables dont exists anymore)

A new variation variable is introduced: @variationHeadingLegacyHeader (default false)
When set to true, it will generate the old .header classes in addition to the .heading classes for easier migration (but will double all selectors which increases css filesize)

This is a HUGE BREAKING CHANGE as

• custom themes won't work when they override @header... variables as those do not exist anymore
• headings won't be built anymore unless "header" is renamed to "heading" in each custom theme.config and each custom semantic.json
• existing projects wont render headings properly unless all header usages are replaced by heading class usages

Still to be decided / taken into consideration

I leave this PR as draft for now as we still have to check/decide how to deal with other "header" usages in other components:

• The th tag in a table is still a "header", not a "heading". However the table component shares the same @heading variables as the heading element itself. This is inconsistent, i think there are dedicated new variables needed
• There are accordion, popup, message, toast, modal, flyout, list item, menu item, ... which all have/use a header (which either is a "heading", a "title" or still a "header"

If this gets merged, we may announce all upcoming breaking changes (espcially this one) long before 2.10.0 gets released, so people are prepared.