fix fields with names containing backticks being inaccessible

[zerbina]

Summary

Fix a regression where field names with backticks in them couldn't be accessed with a field access.

Fixes https://github.com/nim-works/nimskull/issues/1379.

Details

• identifier nodes resulting from gensyms are now tagged with a new node flag (nfWasGensym)
• originalName only strips the gensym suffix from identifiers marked with the flag
• the node flag is persistent, so that it stays on the node across tree copies

Looking for the full "`gensym" suffix wouldn't work, because it would also trigger the stripping for user-created names containing the suffix.

[saem]

/merge

[github-actions[bot]]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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## Notes for Reviewers * an interim solution until symbol binding/lookup for templates is overhauled, at which point the node flag becomes obsolete again * a regression introduced by #1298