Split off SkylarkSemanticsOptions into an immutable class

This is a first step toward making the core Skylark interpreter (the syntax/ directory) not depend on the options parser. Subsequent CLs will replace uses of SkylarkSemanticsOptions within the interpreter with uses of SkylarkSemantics, and move SkylarkSemanticsOptions to the packages/ directory alongside SkylarkSemanticsCodec.

SkylarkSemantics will also replace SkylarkSemanticsOptions as the value that gets passed through Skyframe. This is nice because SkylarkSemantics is strictly immutable, whereas options classes are only kinda-sorta-immutable.

The downside is significantly more redundancy when defining new options. But some of the work is saved by using AutoValue, and there are tests that protect us from dumb mechanical errors. The details are outlined in the javadoc for SkylarkSemanticsOptions and SkylarkSemanticsConsistencyTest.

RELNOTES: None
PiperOrigin-RevId: 171060034