Add syntax_version identifier to molecule

[eval-exec]

This PR want to make molecule support syntax version identifier.

We can specify syntax = {version} now.

1. If syntax_version wasn't specified, it would be 1 by default.
2. All schema file you imported must have same version : a syntax = 1 schema file can't import syntax = 2 file, and syntax = 2 file can't import syntax = 1 schema file either.

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@yangby-cryptape @driftluo @code-monad @zhangsoledad Invite you to review this PR.

[doitian]

Could you explain the motive to add syntax version?

[eval-exec]

Could you explain the motive to add syntax version?

It's like Protocol Buffers' syntax identifier, https://protobuf.dev/reference/protobuf/proto3-spec/#syntax

The "molecule primitive types #62" feature will add uint{8,16,32,64}, int{8,16,32,64} and bool as primary types, so we need the syntax identifier to tell molecule whether to treat these types as primitive types or not.

[eval-exec]

Rebased on master branch. Ready for review. @quake