Support embedded fields in OnConflict for MySQL

[mgibowski]

Fixes #1907

Problem

MySQL dialect generates wrong SQL for embedded properties in onConflictUpdate. Correct SQL fragment would be: field = VALUES(field) and currently it is field = field.

Solution

I have extended pattern matching in conflict tokenizer to support nested excluded property and added corresponding test to MySQLDialectSpec.

Notes

• This solution supports only one level of nesting embedded classes. Is it enough for real-world use cases? If not, one could add more cases in the pattern matching clause.
• I checked implementations of other dialects supporting OnConflict (Postgres and Sqlite) and they both work fine. If necessary, I could add passing tests that verify that.

Checklist

• [x] Unit test all changes
• [x] Update README.md if applicable
• [x] Add [WIP] to the pull request title if it's work in progress
• [x] Squash commits that aren't meaningful changes
• [x] Run sbt scalariformFormat test:scalariformFormat to make sure that the source files are formatted

@getquill/maintainers

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[guizmaii]

@mgibowski Can you rebase your PR, please?