Add implicit block content tooling

[charlespwd]

In this PR

Treat content as the always-available implicit string parameter inside theme block files.

Problem

Block calls already receive built-in content from an explicit argument or inline body, but block files did not expose that contract consistently. Bare content was reported as undefined, block.content had no useful type information, and a schema setting could declare content with an incompatible type without an error-severity check.

Solution

Inside blocks/*.liquid, the language server now types both bare content and block.content as string. They receive the same completion and hover behavior, and later assignments can still change the bare variable's type from that point in the file. Section-side block objects are unchanged.

Theme Check adds two recommended checks:

• BlockContentUsage reports an error on block.content and directs authors to bare content.
• ValidBlockContentSettingType reports an error when a schema setting named content maps to a non-string Liquid type.

String-compatible schema settings and LiquidDoc requiredness continue to merge with the built-in parameter. Caller syntax is unchanged: plain content: remains valid and caller block.content: remains invalid.

This follows #1308 and includes changesets for Theme Check Common, Theme Check Node, and Theme Language Server Common.