GSP: "Split current tag" intention could be implemented for groovy tags

Currently, the intention called from a groovy tag body splits enclosing html tag. It is confusing and leads to uncompilable code.
Before:
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*Imported from [IDEA-54208](https://youtrack.jetbrains.com/issue/IDEA-54208) · Type: Feature · Votes: 0*
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Start by locating the implementation of the “Split current tag” intention and its handling for Groovy/GSP tags, then reproduce the before-and-after examples in the issue. Done means invoking the intention inside a Groovy tag body splits the appropriate inner content without splitting the enclosing HTML tag or producing uncompilable markup.

Written by the indexing model from the issue text.