Define language injections for gsp-tags

Now an injection for xml-tag works in gsp, but you can not configure the injection for gsp-tags. Such as injection tag <g:javascript> hard-coded in grails-plugin.

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*Imported from [IDEA-101164](https://youtrack.jetbrains.com/issue/IDEA-101164) · Type: Feature · Votes: 1*
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Start by inspecting the hard-coded injection for gsp-tags in grails-plugin and the existing xml-tag injection handling. Trace how IntelliJ language injections are configured, then verify that an injection such as can be configured for gsp-tags rather than being fixed in the plugin.

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