fix(compiler): support decode all HTML entities in the value of prop and attribute (fix #8895)

[JuniorTour]

Profit：

1. Close Non-breaking space acts different than other characters - outputs " " in template when passed via props #8895, HTML entities in attributes are not always decoded for string templates #8805, entities in attribute are unexpectedly encoded #10121 and many issues about decoding HTML entities.
2. Go a step further to decode all the HTML entities in the value of prop and attribute. Ensuring consistent behavior between Vue.js and browser. Solving the HTML entities decode issue once and for all.
3. Optimizing the particular logic of fixing :style with multiline attribute removes root element in all versions of IE #3663 and Unicode added in HREF / SRC link (only on Chrome) #6828, simplify the code, improve the robustness.

DEMO:

	Before Fix	After Fix
Reproduction Link	https://jsfiddle.net/juniortour/op1L4z3s/	https://jsfiddle.net/juniortour/qg9pwz7v/
Screenshot

• Fix Non-breaking space acts different than other characters - outputs " " in template when passed via props #8895
• Fix HTML entities in attributes are not always decoded for string templates #8805
• Fix entities in attribute are unexpectedly encoded #10121

---

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: