Show a warning if there is version mismatch between `swiftly` and Xcode toolchains

[rbenegal]

Is your feature request related to a problem? Please describe.
In some cases where the user has different swiftly and Xcode swift toolchains, the user may see cryptic errors when trying to build or run a project. The following is example of the vapor project, trying to build/run with mismatched version:

Describe the solution you'd like
We should detect the case where there is a version mismatch and advise the user that they should either update their swiftly install or get a new version of Xcode.

[Flamki]

Opened PR #2109 to address this.

Summary of the implemented approach:

• Added detection for likely Swiftly/Xcode toolchain mismatch failures on macOS using command output signals (mixed Swiftly + Xcode toolchain paths, known mismatch/failure messages, and differing swiftlang-* versions).
• Added a targeted warning that explains the likely mismatch and offers actions:
  • Select Toolchain
  • Open Documentation (supported toolchains docs)
• Wired this into both:
  • package loading failures (swift package describe path)
  • Swift task output used in build/test/run flows
• Added one-time-per-folder/session deduping to avoid repeated warnings.
• Added unit tests for detection and warning actions.

PR link: #2109