Closes #1521.

I've dreamed of this being possible for over a decade. Now it's a reality.

-- Tired
SILE.call("noindent")
SILE.call("font", { family = "Libertinus Serif" }, content)

-- Wired
noindent()
font({ family = "Libertinus Serif" }, content)

I'm still considering what command scopes are useful where. Having this available highlights some issues to think through:

• How to handle XML vs SIL vs Lua global namespacing rules.
• How to surface useful things like SILE.typesetter:typeset(), register a thin wrapper aronud it?
• Should this be made available by default inside Lua module code?
• How can we handle linting witih such a range of commands available?
• What should be do about document commands that clobber Lua globals?