feat: describe schemas in the DSL

[MShahzaib]

Closes #13.

Summary of Changes

• added "schema" declaration to grammar for describing a dataset's initial schema
• added ".sdsschema" extension for schemas

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CREDENTIALS	secretlint	yes		no	2.97s
✅ GIT	git_diff	yes		no	0.01s
✅ JSON	eslint-plugin-jsonc	1	0	0	1.42s
✅ JSON	jsonlint	1		0	0.35s
✅ JSON	prettier	2	0	0	1.07s
✅ JSON	v8r	1		0	5.49s
✅ KOTLIN	ktlint	8	0	0	22.45s
✅ MARKDOWN	markdownlint	1	0	0	0.58s
✅ MARKDOWN	markdown-link-check	1		0	0.72s

See errors details in artifact MegaLinter reports on CI Job page
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[lars-reimann]

Very thorough work, just two small changes required (see other comments).

Also, for the future: Please also keep larger refactorings that are unrelated to the current feature (like changing the names of predicates to lowercase) in a separate PR. It's alright this time, though.

[lars-reimann]

LGTM.