/utils: add all missing `String` generics

[mindrones]

• investigate which functions of [1] are not covered by functions accepting iterables in lamb or @svizzle/utils

  • for those having a correspondent function in @svizzle/utils [2], make sure they have @see pointing to the correspondent String method

  • provide the missing ones as generics (e.g. toLowerCase) with tests as usual so that we can see how they work

• for all of them, make sure their name explicitly refer to a string input (e.g. split seems too generic)

(if needed, split this task in 2+ PRs)

[1] Possibly missing String generics

String.prototype.charAt()
String.prototype.charCodeAt()
String.prototype.codePointAt()
String.prototype.concat()
String.fromCharCode()
String.fromCodePoint()
String.prototype.includes()
String.prototype.indexOf()
String.prototype.lastIndexOf()
String.prototype.localeCompare()
String.prototype.match()
String.prototype.matchAll()
String.prototype.normalize()
String.prototype.padEnd()
String.prototype.padStart()
String.raw()
String.prototype.repeat()
String.prototype.replace()
String.prototype.replaceAll()
String.prototype.search()
String.prototype.slice()
String.prototype.substring()
String.prototype.toLocaleLowerCase()
String.prototype.toLocaleUpperCase()
String.prototype.toLowerCase()
String.prototype.toString()
String.prototype.toUpperCase()
String.prototype.trimEnd()
String.prototype.trimStart()
String.prototype.valueOf()

[2] Existing String generics

method	current name	new name
String.prototype.endsWith	endsWith	stringEndsWith
String.prototype.slice	slice	sliceString
String.prototype.split	split	splitString
String.prototype.startsWith	startsWith	stringStartsWith
String.prototype.trim	trim	trimString