[Markdown] Simplify frontmatter punctuation scopes (#3941)

This commit renames scopes of `---` frontmatter markers to:

   meta.frontmatter punctuation.section.frontmatter.[begin|end]

Relational
---

1. Frontmatter is not scoped `meta.block` and probably shouldn't be.
   Thus using `punctuation.section.block` seems not a perfect match 
   for scoping `---` markers.

2. `punctuation.section.block` scope is primarily used 
   to denote `{` and `}` in C like syntaxes.

3. The change intents to apply scope naming guideline which 
   uses `meta.<name> punctuation.section.<name>.[begin|end]` scheme.

4. Only few syntaxes extend punctuation scopes after `begin` or `end`.

Markdown/Markdown.sublime-syntax

@@ -311,25 +311,25 @@ contexts:
     - match: (---)\s*(?i:(json)\s*)\n
       captures:
         0: meta.frontmatter.markdown
-        1: punctuation.section.block.begin.frontmatter.markdown
+        1: punctuation.section.frontmatter.begin.markdown
         2: constant.other.language-name.markdown
       embed: scope:source.json
       embed_scope: meta.frontmatter.markdown source.json.embedded.markdown
       escape: ^(---|\.{3})\s*\n # pandoc requires the remainder of the line to be blank
       escape_captures:
         0: meta.frontmatter.markdown
-        1: punctuation.section.block.end.frontmatter.markdown
+        1: punctuation.section.frontmatter.end.markdown
     - match: (---)\s*(?i:(yaml|yml)\s*)?\n
       captures:
         0: meta.frontmatter.markdown
-        1: punctuation.section.block.begin.frontmatter.markdown
+        1: punctuation.section.frontmatter.begin.markdown
         2: constant.other.language-name.markdown
       embed: scope:source.yaml
       embed_scope: meta.frontmatter.markdown source.yaml.embedded.markdown
       escape: ^(---|\.{3})\s*\n  # pandoc requires the remainder of the line to be blank
       escape_captures:
         0: meta.frontmatter.markdown
-        1: punctuation.section.block.end.frontmatter.markdown
+        1: punctuation.section.frontmatter.end.markdown
 
   markdown:
     - include: indented-code-blocks