Add custom structfield linter to check struct field names and tags

[alexandear]

This PR renames jsonfieldname linter to structfield and extends it to check struct tags.

The linter enforces that:

• The tag names for json and url match the struct field name
• Fields with json and url tags that include omitempty must use pointers types, except for slices, maps, json.RawMessage, and any.

Example output for the ListCursorOptions struct:

go-github/github/github.go

Lines 262 to 267 in 671b8b0

	typeListCursorOptionsstruct {
	// For paginated result sets, page of results to retrieve.
	Pagestring`url:"page,omitempty"`
	// For paginated result sets, the number of results to include per page.
	PerPageint`url:"per_page,omitempty"`

$ ./script/lint.shgithub/github.go:264:7: change the "Page" field type to "*string" in the struct "ListCursorOptions" because its tag uses "omitempty" (structfield) Page string `url:"page,omitempty"` ^github/github.go:267:10: change the "PerPage" field type to "*int" in the struct "ListCursorOptions" because its tag uses "omitempty" (structfield) PerPage int `url:"per_page,omitempty"` ^...

Closes#3775

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.42%. Comparing base (903265b) to head (e83e366).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3843 +/- ##
=======================================
Coverage 92.41% 92.42% =======================================
Files 197 197 Lines 14152 14167 +15 =======================================
+ Hits 13079 13094 +15 
Misses 884 884 Partials 189 189 

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[gmlewis]

Cool! Thank you, @alexandear!
Just a couple questions, really.

[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.