Add support for various inline styles

[tneotia]

Fixes #516, implements everything that was WIP in #228.

Implements direction, display, font-feature-settings, font-family, font-style, font-size, font-weight, line-height, text-decoration, text-decoration-line, text-decoration-color, text-decoration-style, and text-shadow support based on CSS guidelines.

Added Display.NONE to the existing Display enums to allow style="display: none;"

Preview:

Code for above example:

<p style="direction: rtl;">This text should go from right to left</p>
<p style="display: none;">This text should not show up</p>
<p style="font-feature-settings: smcp 1;">This text's lower case letters should be small capital</p>
<p style="font-family: times;">This text should be in Times New Roman</p>
<p style="font-style: italic; font-family: monospace;">This text should be italic and in monospace</p>
<p style="font-weight: bold;">This text should be bold</p>
<p style="line-height: 3">This is going to be a really long text element just to test line height which I have set in the style of this paragraph to be three times the height of this paragraph's font size.</p>
<p style="line-height: 100%">This is going to be a really long text element just to display the default line height which is the same as the height of this paragraph's font size multiplied by 1.2 as per CSS spec.</p>
<p style="text-shadow: 2 2, 0 0 5px #DE6721;">This text should have a really weird text shadow</p>
<p style="text-decoration: underline overline line-through #DE6721 wavy;">This text should have some fancy text decorations</p>

This is going to need some pretty rigorous testing imo, there's a lot of different variables to consider when creating the inline style implementations so I want to make sure as many edge-cases are covered as possible.

[erickok]

Good work. Just slightly confusing variable naming here for me.

[erickok]

This doesn't do anything right? (it creates a new list, bu you don't assign it to anything).

[tneotia]

It removes duplicates from the list. Kind of odd, but it works nicely and is super concise.

[erickok]

I thought that's what you wanted, but it doesn't, because the list is not modified in-place.

    test("toSet_inline", () async {
      final list = ["one", "two", "one"];
      list.toSet().toList();
      expect(list, ["one", "two"]);
    });

Will fail:

Expected: ['one', 'two']
  Actual: ['one', 'two', 'one']

But this works of course:

    test("toSet_inline", () async {
      final list = ["one", "two", "one"];
      expect(list.toSet().toList(), ["one", "two"]);
    });

[tneotia]

I thought it was modified in place. I will fix this as well.

[tneotia]

Fixed.

[erickok]

Same here - this has no effect right?

[tneotia]

It removes duplicates from the list here as well.

[tneotia]

Fixed.

[erickok]

Would have preferred to have the units as optional param, with '' as default.

Alternatively, use named constructors for perc, px and rm?

[tneotia]

I should have made it optional... don't know what I was thinking I was probably a little tired 😅 I'll do the same for LineHeight.

[tneotia]

Fixed.