feat(core): include jsii runtime version in analytics

[eladb]

Add the jsii runtime version to the library version reporting under "jsii-runtime".

jsii runtime clients populate the JSII_AGENT environment variable with this information (see aws/jsii#325). If JSII_AGENT is not defined, we assume this is a node.js runtime and include the node.js version.

Fixes #1258
Fixes awslabs/cdk-ops#127

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[RomainMuller]

👌🏻

[mergify]

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