refactor: Moved filter composer files to src/composer [#1469]

This MR contains the following changes:

• Moved filter composer files to src/composer/ directory.
  • Updated file references accordingly.
  • Updated build configuration accordingly.
• Applied coding style changes.
  • Added void prefix to async function calls.
• Moved initialization logic into start() function.
  • Moved module globals to the beginning of the file.
• Disabled type checking for the time being (see below).

This MR does not contain the following changes:

• Converted modules to TypeScript.
• Use await to replace promise chains.
• Replace browser.runtime.sendMessage() call with api.* API.
• Refactored logic or code structure (e.g. doInit() function could be dissolved in favor of start()).
  Changes to the logic and code structure should be tackled in a separate issue following up to this one.