Enable configuring Dart's LSP from other means of installation types.
Some users don't install the `dart` binary, but uses version manager.
In the example, I uses [FVM](https://fvm.app/) (short for "Flutter
Version Manager").
I have tested this with "Install Dev Extensions".
Release Notes:
- N/A
cc other maintainer: @agent3bood @flo80
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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
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