LSP Completion: use textEdit.newText as a workaround

Merged oldherl oh requested to merge oldherl/kate:oldherl/use-newText into master

Some LSP server implementation, such as rust-analyzer 2021-11-01, do not use insertText field, but uses textEdit.

Since we don't have real support for textEdit (see Commit 831a0069), the field textEdit.newText is used as a workaround here to prevent a worse fallback to use label as the inserted text.

BUG: 445085

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