fix(core): drop the toBoolean function

So far Angular have used the toBoolean function to decide if the parsed value
is truthy. The function made more values falsy than regular JavaScript would,
e.g. strings 'f' and 'no' were both treated as falsy. This creates suble bugs
when backend sends a non-empty string with one of these values and something
suddenly hides in the application

BREAKING CHANGE: values 'f', '0', 'false', 'no', 'n', '[]' are no longer
treated as falsy. Only JavaScript falsy values are now treated as falsy by the
expression parser; there are six of them: false, null, undefined, NaN, 0 and "".

Fixes #3969
Fixes #4277

cc @IgorMinar @petebacondarwin
actually, it catches both when `attr.ngTrim === undefined` (not `false`) and when `attr.ngTrim === 'false'`. `attr.ngTrim` is either undefined or a string, like all `attr` values.

So yeah, we're not parsing it (I don't know why), so we have to sort-of parse it here.

Btw, that also means `ng-trim="some_falsy_variable"` won't work, why is it implemented in this way?