Add a parser

[pjhartzell]

Adds a parser for the MWL language. A CLI and extended documentation in the hugo website are also included in this PR.

The parser is the key feature. It operates on an MWL document in three passes, each consuming the previous pass's output, with a single diagnostic list that all passes append to.

1. decode — turns the source bytes into a value tree: the whole document held in a single struct, Value, which represents any JSON value (null, boolean, number, string, object, array) and nests instances of itself — an object holds its members as name/Value pairs, an array holds its elements as Values — so the document forms a tree whose nodes are Values. Every value and every object key is tagged with its byte offset, line, and column, and object members are kept as an ordered slice rather than a map, so source order survives and duplicate keys are detectable. Each node is labeled only by its JSON type; no MWL meaning is attached yet. This pass rejects malformed JSON, empty documents, duplicate object members, and trailing content. No later pass touches the bytes.

2. bind — walks the value tree and builds the typed AST, whose nodes are the language's own shapes (Flow, Step, Call); this is where MWL meaning first exists. Shape is enforced during the translation itself, so no AST node is ever built from a malformed value and nothing downstream re-checks: each value must be the JSON type its field requires (next must be a string, steps an object), objects may carry only the members their shape accepts (and must carry the required ones), exactly-one pairs (a call's provider or flow, a Sleep's for or until) must have exactly one side present, and expressions may appear only where permitted. A violation becomes a diagnostic and binding continues, so one parse reports every shape problem in the document.

3. check — runs the rules that span the bound document, which no single field's shape can express. Every entrypoint and next must resolve to a step; a call's flow name must resolve to a flows declaration, looked up innermost-outward through the enclosing Flows the way variable lookup walks nested scopes; and the flow-reference graph must be acyclic. It also produces all of the parser's warnings, where a document is legal but rarely what the author meant: a flows entry that shadows an outer declaration of the same name, a failure-catalog entry whose code sits outside the provider's own kind and prefix, a structural declaration naming no parameter of its phase (so it marks nothing). It runs only on an AST that bound without errors.

See also the README.md documents in the parser and cmd/mwl modules. Launch the hugo site locally to view an AST explorer.