Enable @typescript-eslint/explicit-function-return-type ESLint rule

[ficristo]

• Enabled explicit-function-return-type for some folders #296: Enabled explicit-function-return-type for some folders
• Enabled explicit-function-return-type for command folder #299: Enabled explicit-function-return-type for command folder
• Enabled explicit-function-return-type for document folder #300: Enabled explicit-function-return-type for document folder
• Enabled explicit-function-return-type for search folder #302: Enabled explicit-function-return-type for search folder
• Enabled explicit-function-return-type for CodeFolding extension #304: Enabled explicit-function-return-type for CodeFolding extension
• Enabled explicit-function-return-type for some files in languageTools folder #305: Enabled explicit-function-return-type for some files in languageTools folder
• Enabled explicit-function-return-type in app folder #319: Enabled explicit-function-return-type in app folder
• Enabled explicit-function-return-type in features folder #320: Enabled explicit-function-return-type in features folder
• Enabled explicit-function-return-type in utils folder #321: Enabled explicit-function-return-type in utils folder
• Enabled explicit-function-return-type in language folder #322: Enabled explicit-function-return-type in language folder
• Enabled explicit-function-return-type in editor folder #323: Enabled explicit-function-return-type in editor folder
• Enabled explicit-function-return-type in view folder #324: Enabled explicit-function-return-type in view folder
• Enabled explicit-function-return-type in filesystem folder #325: Enabled explicit-function-return-type in filesystem folder
• Enabled explicit-function-return-type in JSUtils folder #328: Enabled explicit-function-return-type in JSUtils folder