fix(build): use invariantSeparatorsPath for PROJECT_DIR to ensure cross-platform consistency

[PeterUlb]

Motivation and Context

On windows, the task :smithy-rs:codegen-core:compileKotlin fails:

> Task :smithy-rs:codegen-core:compileKotlin FAILED
e: file:///C:/Users/.../.../smithy/smithy-rs/codegen-core/build/generated/src/main/kotlin/BuildEnvironment.kt:6:40 Unsupported escape sequence.

This means I cannot run the task to generate the server sdk crate on windows (smithyBuild)

Example generated file:

object BuildEnvironment {
    const val MSRV: String = "1.86.0"
    const val PROJECT_DIR: String = "C:\Users\...\smithy\smithy-rs"
}

Description

Using invariantSeparatorsPath to use the invariant separator '/' to separate the names in the name sequence. The forward slashes as seperator still work on windows.

Testing

Executed task on windows and inspected BuildEnvironment.kt files. Now has forward slashes.

object BuildEnvironment {
    const val MSRV: String = "1.86.0"
    const val PROJECT_DIR: String = "C:/Users/.../smithy/smithy-rs"
}

Checklist

Not applicable

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