feat(md077): support fixed continuation indent config

[DavidPandleton]

Adds an optional indent setting to MD077 so documents that follow a different indentation convention (4 spaces for MkDocs, or a .editorconfig-driven style) can pin the required continuation indent instead of the content-column-derived default. When set, the requirement becomes marker column + indent, per item.

Closes #784.

[rvben]

Thanks for this, and for the clean implementation. The per-item marker column + indent shape is right, and I checked it against the document in #784: with indent = 4, rumdl fmt produces exactly the 4-space form that was asked for, and a re-check is clean.

I merged this and pushed four corrections on top rather than sending you round-trips, since most of them need repo internals that are not reasonable to expect from outside:

1. The config key validator did not know about indent. impl_rule_config_methods!(MD077Config) derives a rule's known keys by serializing the default config through JSON to TOML, which drops None fields, so an Option key is invisible to it. [MD077] indent = 4 printed Unknown option for rule MD077: indent and failed the run under --deny-config-warnings, even though the setting itself worked. The nullable arm of that macro fixes it. Worth knowing for any future Option config key.
2. Under --flavor mkdocs, a configured indent could lower the 4-space minimum. With indent = 2, a document correctly indented at 4 was reported as over-indented and fix rewrote it to 2, which Python-Markdown no longer reads as part of the list item. That minimum is a compatibility floor rather than a style default, so a configured indent may now raise it but not lower it. It stays scoped to that flavor: under standard, indent = 2 still means 2.
3. One message claimed a consequence that was not true. A configured indent produced "needs 4 spaces of indentation to remain part of the list" for content that was still part of the list. A requirement coming from the configured indent now says "to match the configured indent"; the structural and MkDocs wordings are unchanged.
4. indent = 0 is now rejected. It asks for continuation at the marker's own column, which ends the list item, so correctly indented content was flagged as "over-indented (expected 0, found 2)".

I also corrected docs/md077.md, which said content below the requirement is flagged under the same rules as the default. That is not true of tight lazy continuation: style still decides which lines get checked, and indent only decides what they are checked against.

Thanks again.