Implement trimStart/trimEnd

[dilijev]

Ported from OS#18780560

Stage 3 Proposal: https://github.com/tc39/proposal-string-left-right-trim
We already have implementations of trimLeft and trimRight, so we simply need to add trimStart and trimEnd as aliases to those.

>es --tags jsvu-web -is E:\work\bugs\19h1\trimStart-trimEnd\trimStart-trimEnd.js
## Source
let stringToTrim = '  hello  '
function trimIt(trimmerFn) {
    return '.' + trimmerFn.call(stringToTrim) + '.';
}
function has(funcName) {
    let doesHave = String.prototype.hasOwnProperty(funcName);
    let doesHaveString = doesHave ? '++ yes ++' : '-- NO --'
    print(`has ${funcName}? ${doesHaveString}`);
    if (doesHave) {
        print(trimIt(String.prototype[funcName]))
    }
}
has('trimLeft')
has('trimRight')
has('trimStart')
has('trimEnd')

#### jsvu-ch
has trimLeft? ++ yes ++
.hello  .
has trimRight? ++ yes ++
.  hello.
has trimStart? -- NO --
has trimEnd? -- NO --
#### jsvu-jsc, jsvu-sm, jsvu-v8
has trimLeft? ++ yes ++
.hello  .
has trimRight? ++ yes ++
.  hello.
has trimStart? ++ yes ++
.hello  .
has trimEnd? ++ yes ++
.  hello.

[dilijev]

Another set of test cases:

>eshost --tags jsvu-web -is test.js
## Source
let assert = {
    areEqual(a, b) {
        if (a !== b) {
            print(`FAIL: expected ${a} === ${b}`);
        }
    }
}

assert.areEqual(String.prototype.trimLeft, String.prototype.trimStart);
assert.areEqual(String.prototype.trimRight, String.prototype.trimEnd);

if (String.prototype.trimStart) {
    assert.areEqual(String.prototype.trimStart.name, 'trimStart')
} else {
    print("FAIL: String.prototype.trimStart should not be undefined")
}

if (String.prototype.trimEnd) {
    assert.areEqual(String.prototype.trimEnd.name, 'trimEnd')
} else {
    print("FAIL: String.prototype.trimEnd should not be undefined")
}

assert.areEqual(String.prototype.trimLeft.name, 'trimStart')
assert.areEqual(String.prototype.trimRight.name, 'trimEnd')

print("DONE")


#### jsvu-ch
FAIL: expected function trimLeft() { [native code] } === undefined
FAIL: expected function trimRight() { [native code] } === undefined
FAIL: String.prototype.trimStart should not be undefined
FAIL: String.prototype.trimEnd should not be undefined
FAIL: expected trimLeft === trimStart
FAIL: expected trimRight === trimEnd
DONE

#### jsvu-jsc, jsvu-sm, jsvu-v8
DONE