feat(core): make multiline env, newline breaks and bar rendering configurable

[iasandcb]

Depends on #21. That commit is included here and will drop out once it merges.

Three rendering behaviours are hardcoded with no way to opt out:

Behaviour	Location
multiline expressions wrapped in aligned	codegen.ts
a blank line (two \n) required to emit \\	trie.ts
an unpaired | rendered as \mid	parser.ts

Each is a reasonable default, but each is also a matter of house style. I
maintain an Obsidian plugin whose vault convention differs on all three, and
the only way to get there today is to patch the built bundle.

Change

Three new optional fields on AsciiMathConfig:

new AsciiMath({
  multilineEnv: 'gather*',   // default 'aligned'
  singleNewlineBreak: true,  // default false
  barAsMid: false,           // default true
})

All three default to the existing behaviour, so this is backwards compatible.

Implementation notes

• codegen takes an options object as a second argument and threads it
  through its own recursion.
• Trie gains a newline threshold (2 or 1) set from singleNewlineBreak.
• barAsMid is read from a module-scoped variable that parser() sets on
  entry. walk recurses through roughly 30 call sites and threading an
  options object through all of them would be a large diff for a single
  boolean. parser is synchronous and non-reentrant so this is safe, but if
  you would rather see it threaded explicitly I am happy to redo it — it is
  mechanical, just noisy.

A fourth customisation I needed — norm without \left/\right —
required no core change at all; the existing symbols extension point covers
it. That may be worth a line in the README, since it is not obvious that
built-in tokens can be overridden and not just extended.

Tests

test/render-options.test.ts, 11 cases covering defaults, each option
individually, and all four composed. One snapshot line changes.

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