Document links

[wkillerud]

Feature solved in #1, we could still do with some automated tests. Example test cases and fixtures:

• https://github.com/wkillerud/some-sass/blob/35cd68d30100f28d50b83482aa82eddd2c2d74bb/packages/vscode-css-languageservice/src/test/sass/sassNavigation.test.ts#L709
• https://github.com/wkillerud/some-sass/blob/main/packages/language-services/src/features/__tests__/find-document-links.test.ts
• https://github.com/wkillerud/some-sass/blob/main/vscode-extension/test/e2e/defaults-scss/navigation.test.js
• https://github.com/wkillerud/some-sass/tree/main/packages/vscode-css-languageservice/test/linksTestFixtures