BREAKING(path/unstable): move unstable overload of basename to unstable-basename

[kt3k]

part of #5920

This PR moves the unstable overload of basename to unstable-basename

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.32%. Comparing base (d1f5a36) to head (1cf02dd).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
path/posix/unstable_basename.ts	90.00%	1 Missing ⚠️
path/windows/unstable_basename.ts	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5957   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         484      486    +2     
  Lines       39403    39425   +22     
  Branches     5837     5836    -1     
=======================================
+ Hits        37953    37976   +23     
+ Misses       1408     1407    -1     
  Partials       42       42           

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[iuioiua]

Suggested change

	export function basename(path: string, suffix?: string): string {
	export function basename(path: string, suffix = ""): string {

[iuioiua]

Ditto for the other functions

Suggested change

	export function basename(path: string, suffix?: string): string {
	assertArgs(path, suffix ?? "");
	export function basename(path: string, suffix = ""): string {
	assertArgs(path, suffix);