gopls/hover: show named func type doc when hovering over func literal

[abhay1999]

When a func literal is implicitly converted to a named function type
(e.g. fs.WalkDirFunc), hovering over its "func" keyword now shows the
documentation and signature of that named type, with a link to
pkg.go.dev.

Previously, hovering over "func" in:

filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
	...
})

showed only the anonymous signature. Now it shows the docs for
fs.WalkDirFunc, which is often the information the user actually wants.

Implementation: when cur.Node() is *ast.FuncType with the cursor at the
"func" token and the parent is a *ast.FuncLit, the new hoverFuncLit
function calls typesutil.TypesFromContext to find the named function
type the literal must satisfy, loads the declaring package via
NarrowestDeclaringPackage, and returns a hoverResult with the named
type's documentation, signature, and pkg.go.dev link. If no named type
is found, it falls back to the existing anonymous-type hover behavior.

Note: FindByPos at a FuncLit's "func" token returns the inner
*ast.FuncType node (not *ast.FuncLit), so the code checks the parent
node to detect the FuncLit context.

A marker test is added in hover/funclittype.txt covering both the
named-type case and the fallback case.

Fixes golang/go#76191

[gopherbot]

This PR (HEAD: 82a1fea) has been imported to Gerrit for code review.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Abhay Chaurasiya:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/764120.
After addressing review feedback, remember to publish your drafts!