rename src/class-fields to src/class-elements

src/class-elements/call-expression-privatename.case

src/class-elements/class-evaluation-error/cls-decl.template

src/class-elements/class-evaluation-error/cls-expr.template

src/class-elements/computed-name-referenceerror.case

src/class-elements/computed-name-toprimitive-err.case

...ements/computed-name-toprimitive-returns-noncallable.case

...elements/computed-name-toprimitive-returns-nonobject.case

src/class-elements/computed-name-toprimitive-symbol.case

src/class-elements/computed-name-toprimitive.case

src/class-elements/computed-name-tostring-err.case

src/class-elements/computed-name-valueof-err.case

src/class-elements/computed-names.case

src/class-elements/computed-symbol-names.case

src/class-elements/conventions.md

src/class-elements/ctor-called-after-fields-init.case

src/class-elements/default/cls-decl.template

src/class-elements/default/cls-expr.template

...ments/delete-error/cls-decl-field-delete-covered.template

...delete-error/cls-decl-field-delete-twice-covered.template

...lass-elements/delete-error/cls-decl-field-delete.template