Add ToC & max clause depth to provide flexibility

[gesa]

Ecma house style is a ToC 3 deep and max clauses 6, but there may be scenarios where a TC wants to generate a draft with fewer or more in the context of their specific standard

[gesa]

I received clarification on conflicting direction from the secretariat. The maximum total clause depth is 5 (1.2.3.4.5), not 5 subdivisions. I have updated the PR in https://github.com/tc39/ecmarkup/pull/663/commits/7c88604659de82f77f77d00f69780439902bbdeb