v.utils: allow to set names of compared items when diffing strings

[ttytm]

Allows further customization when comparing strings.

It adds the ability to set the names that are used for the temporary files when comparing strings directly.

Two main reason:

• Some names in the output can fit better
• Several diff viewers colorize based on on the file extension. E.g. when using delta or piping output like v fmt -diff file.v | diff-so-fancy or using other viewers set via VDIFF_CMD.

Filename samples:

Current default (not customizable)	Updated default	Updated names passed

E.g. using vs not using file extensions:

import v.util.diff

fn main() {
	txt1 := '
sum = num1 + num2
print("The sum of {0} and {1} is {2}".format(num1, num2, sum))
	'
	txt2 := '
sum = num1+num2
print("The sum of {0} and {1} is {2}".format(num1, num2, sum))
	'
	println(diff.compare_text(txt1, txt2)!)
	// println(diff.compare_text(txt1, txt2, base_name: 'old.py', target_name: 'new.py')!)
}

filename not set	colored based on filename

Suggestions for other new default names than base and target are welcome. Those should be general names similar to text1 and text2 since it's not always a comparison of e.g. expected and output.

[spytheman]

Can you please make a test for it?