Prepare parser for sum types syntax #2425

Job	Run time
quint-antlr-grammar	47s
quint-linting	47s
quint-examples-dashboard (ubuntu-latest)	4m 31s
quint-examples-dashboard (macos-latest)	7m 37s
quint-integration-tests (ubuntu-latest)	5m 19s
quint-integration-tests (macos-latest)	10m 21s
quint-integration-tests (windows-latest)	6m 37s
quint-test-generated-up-to-date	1m 58s
quint-unit-tests (ubuntu-latest)	1m 23s
quint-unit-tests (macos-latest)	1m 37s
quint-unit-tests (windows-latest)	2m 39s
quint-vscode-linting	45s
quint-vscode-plugin (ubuntu-latest)	2m 40s
quint-vscode-plugin (macos-latest)	7m 46s
quint-vscode-plugin (windows-latest)	4m 6s
	58m 53s

Provide feedback