Add support for range diagnostics under cursor

[satorunooshie]

Added support for range-based diagnostics, aligning with the Language Server Protocol (LSP) Specification.
Previously, diagnostics were matched only to specific start positions, limiting usability for multi-line or range diagnostics.
This update ensures vim-lsp provides functionality consistent with VSCode, which already supports range-based diagnostics.

The highlighted codes represents the area where textDocument/codeAction can be triggered.
Below is the corresponding diff for the changes.

Before

After

VSCode behavior

[satorunooshie]

@prabirshrestha Merry Christmas! 🎄
Just wanted to kindly ping. Let me know if there’s anything I can adjust or clarify.

Thanks, and hope you’re having a great holiday season!

[prabirshrestha]

@satorunooshie can you provide the steps to use. i don't see much difference.

[satorunooshie]

@prabirshrestha Thanks for responding and I'm sorry I'm late.

package main

import "fmt"

func documentDiagnostics() {
	msg := "msg"
	fmt.Printf(msg +
		msg +
		msg,
	)
}

When using a Go language server (e.g., gopls), you might see a warning like:

non-constant format string in call to fmt.Printf

involving lines 7–9. This PR enables vim-lsp to highlight the exact range rather than just the line 7.

Why does this matter?

• Accurate Highlighting: Multi-line warnings/errors get highlighted precisely.

• Better Quickfix & Code Actions: When the diagnostics cover the correct range, automated fixes (e.g., Quickfix or code actions) apply cleanly.

• Consistent with VSCode/LSP spec: This matches the standard Language Server Protocol behavior, which supports start/end positions for diagnostics.