Update the syntax to fix nested languages and emphases

[eugenesvk]

• This fixes broken nested language scopes that didn't stop at the closing tag (e.g., Python)
• Adds a couple of emphasis scopes (underlines/strikethrough)
• Regroups the other scopes
• Adds syntax test (for (nested)emphasis/embedded only) to make it easier to change syntax
• Adds a few more advanced keybinds that, e.g., don't bold already bold text and also are able to Unbold/unitalicize test
• Moves the syntax to version 2 that fixes a bunch of bugs, but requires ST 4075, so you would need to update the package manifest:

If you want to support versions before 4075, then I think you'd need to create either of these:

• two different tag types (e.g., st3-a.b.c and st4-a.b.c) or
• two different branches (e.g., continue to use master for ST<4075 and st4 for later versions)
  and add that info with version ranges to the Package Controls repo, check the examples of MarkdownEditing/Missing Palette Commands:

		{
			"name": "MarkdownEditing",
			"details": "https://github.com/SublimeText-Markdown/MarkdownEditing",
			"homepage": "https://sublimetext-markdown.github.io/MarkdownEditing",
			"labels": ["markdown", "github markdown", "gfm", "multimarkdown", "color scheme"],
			"releases": [
				{
					"sublime_text": "2000 - 3175",
					"tags": "st2-"
				},
				{
					"sublime_text": "3176 - 4106",
					"tags": "3176-"
				},
				{
					"sublime_text": ">=4107",
					"tags": "4107-"
				}
			]
		},

		{
			"name": "Missing Palette Commands",
			"details": "https://github.com/fjl/Sublime-Missing-Palette-Commands",
			"releases": [
				{
					"sublime_text": "<3000",
					"branch": "master"
				},
				{
					"sublime_text": ">=3000",
					"branch": "st3"
				}
			]
		},