Add `puro prepare` to pre-cache flutter artifacts (#143)

[Vonarian]

• introduce puro prepare CLI command with --all-platforms, --platform, and --force flags so users can pre-download Flutter artifacts for a chosen environment
• add environment helper that routes to flutter precache with deterministic platform ordering and sensible defaults per host OS
• document the new command in the manual and changelog to guide usage and highlight the feature

Closes https://github.com/pingbird/puro/issues/143

[Vonarian]

Example:

puro.exe prepare master
Downloading engine information...                                  631ms
Downloading Material fonts...                                    1,133ms
Downloading Gradle Wrapper...                                      212ms
Downloading android-arm-profile/darwin-x64 tools...              2,254ms
Downloading android-arm-release/darwin-x64 tools...              1,923ms
Downloading android-arm64-profile/darwin-x64 tools...            2,183ms
Downloading android-arm64-release/darwin-x64 tools...            2,094ms
Downloading android-x64-profile/darwin-x64 tools...              2,318ms
Downloading android-x64-release/darwin-x64 tools...              1,967ms
Downloading android-x86 tools...                                   23.9s
Downloading android-x64 tools...                                   24.6s
Downloading android-arm tools...                                   24.7s
Downloading android-arm-profile tools...                            9.3s
Downloading android-arm-release tools...                            8.7s
Downloading android-arm64 tools...                                 37.0s
Downloading android-arm64-profile tools...                          9.6s
Downloading android-arm64-release tools...                          9.1s
Downloading android-x64-profile tools...                            9.8s
Downloading android-x64-release tools...                            9.0s
Downloading ios tools...                                           20.2s
Downloading ios-profile tools...                                   16.4s
Downloading ios-release tools...                                   29.8s
Downloading Web SDK...                                              6.6s
Downloading package sky_engine...                                1,177ms
Downloading package flutter_gpu...                                 460ms
Downloading flutter_patched_sdk tools...                         1,765ms
Downloading flutter_patched_sdk_product tools...                 1,701ms
Downloading darwin-arm64 tools...                                   8.3s
Downloading darwin-x64/framework tools...                           7.6s
Downloading darwin-x64/gen_snapshot tools...                        3.3s
Downloading darwin-x64-profile/framework tools...                   4.1s
Downloading darwin-x64-profile tools...                          1,438ms
Downloading darwin-x64-profile/gen_snapshot tools...                3.5s
Downloading darwin-x64-release/framework tools...                  34.1s
Downloading darwin-x64-release tools...                          1,409ms
Downloading darwin-x64-release/gen_snapshot tools...                3.0s
Downloading libimobiledevice...                                    234ms
Downloading libusbmuxd...                                          204ms
Downloading libplist...                                            208ms
Downloading openssl...                                             707ms
Downloading libimobiledeviceglue...                                203ms
Downloading ios-deploy...                                          229ms
Downloading darwin-arm64/font-subset tools...                    1,182ms
[✓] Prepared environment `master` (android, ios, macos, web)

[Vonarian]

Hi @benthillerkus Yes, you're correct, the new formatter changed almost every single file. Though the functionality changes made by this PR are present only on the first couple of commits, the commit messages are mixed up tho :/