Introduce fmt reset (working title prune) and fix JSON frontmatter handling

[sourishkrout]

Most recently lifecycle identity has become entirely optional, we want to provide a command that "resets" markdown documents' lifecycle identity via runme fmt sub commands.

1. runme fmt will continue to leave existing LCID untouched. runme fmt reset will remove any unless the --identity setting (default none) is overwritten.
2. Reset will strip Runme-related frontmatter, leaves unrelated in tact.
3. The PR fixes bugs with JSON-encoded frontmatter.
4. Adds unit tests for all the changes.

[pastuxso]

@sourishkrout, Running runme fmt --identity cell --write, modifies the runme version, is it ok?

diff --git a/frontmatter/README.md b/frontmatter/README.md
index b0d16a5..caca01d 100644
--- a/frontmatter/README.md
+++ b/frontmatter/README.md
@@ -1,7 +1,7 @@
 ---
 runme:
   id: 01HMCJRVF90ED0TXZ0B9D7YYCB
-  version: v2
+  version: v3
 ---

[sonarcloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sourishkrout]

Running runme fmt --identity cell --write, modifies the runme version, is it ok?

Yes, it's supposed to reformat and overwite the major version used at the time.

[pastuxso]

LGTM 🚀