Clarify ReScript licensing (runtime is MIT)

[cknitt]

For broader adoption of ReScript, it is important to have the runtime/stdlib licensed under a permissive Open Source license (MIT) that does not restrict commercial use via the LGPL linking clause.

The compiler itself is derived from the OCaml compiler and therefore licensed under LGPL.

Already licensed under MIT, but not currently labeled as such:

• the ReScript parser under compiler/syntax
• the new standard library (originally from https://github.com/rescript-lang/rescript-core)

For the remainders of the old standard library, including the Belt modules, we contacted the original author Hongbo Zhang and got his ok to relicense them under MIT license, too.

This PR clarifies the licensing situation accordingly and sets the appropriate SPDX license expressions in the various package.json files.

[pkg-pr-new]

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rescript

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@rescript/darwin-arm64

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@rescript/darwin-x64

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@rescript/linux-arm64

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@rescript/linux-x64

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@rescript/runtime

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@rescript/win32-x64

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commit: 8b77400