feat: support imperial system (#125)

* feat: add weight entity and related stuff

- It should simplify the management of the unit system and values

* feat: add user preference in repository and expose it in `CompositionLocal`

* feat: expose user preference in settings screen

- reorganized settings

* refactor: enforce a display of a max number of decimal digits

* feat: resolve default unit system

* docs: expand UserPreferencesRepository documentation

* refactor: move constant in companion

* refactor: mark Weight.kt serializable (for room)

* feat: use Weight value class in Measurement entity

- refactored all code that uses it

* feat: use Weight value class in Set entity for load property

- refactored all code that uses it
- add some factory methods to Weight entity as helpers
- add some docs

* refactor: create custom functions to handle unit system suffix logic automatically and centrally

* perf: cache value across compositions

* docs: update changelog

Author: IamDg

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Support for imperial system (pounds) [#18](https://github.com/LibreFitOrg/LibreFit/issues/18)
 - Russian 🇷🇺 [#124](https://github.com/LibreFitOrg/LibreFit/pull/124)
 - Add 5 new exercises [#111](https://github.com/LibreFitOrg/LibreFit/pull/111):
   - Wall Sit (Isometric squat against the wall)
@@ -18,12 +19,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Jumping Jacks
 
 ### Changed
+
 -
 
 ### Deprecated
+
 -
 
 ### Removed
+
 -
 
 ### Fixed