Internal: Split `Common.ts` to multiple files

[Taitava]

Common.ts will be split to the following new files:

• commonFileSystem.ts
• commonIndependent.ts: Only for functions that do not need to import anything from other files.
• commonObsidian.ts: For functions that import stuff from obsidian.
• commonOthers.ts: For miscellaneous functions that are not eligible for being placed in commonIndependent.ts.
• commonPlatform.ts: For operating system related functions.

These files will be created in a new folder named src/common. These changes should not cause any visible changes for users.

This is done at the same time with #361.