text/template: add the 'parent' keyword

[paulsmith]

Add the keyword parent to the text/template language. The keyword,
when used in place of a template name string in a template execution
node, causes execution of that template to refer to a previously-defined
template of the same name. In the context of template inheritance, this
allows child templates to access the contents of the parent template they
are overriding.

For example:

base template: {{define "title"}Example.com{{end}}

inheriting: {{define "title"}}Welcome - {{template parent .}}{{end}}

Would produce: Welcome - Example.com

Fixes #48266

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Message from Ian Lance Taylor:

Patch Set 1: Code-Review-2

(1 comment)

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[mvdan]

The proposal was declined; closing.