feat: add missing keywords — alias, while, else

[SchoolyB]

Summary

Three keywords from the language spec (STANDARD.md §2.5, §3.5) are missing from the LSP's keyword completion and hover docs:

Keyword	Purpose	Spec Section
alias	Type aliases (alias Meters = float)	§3.5
while	Alias for as_long_as	§6.3.3
else	Alias for otherwise	§6.2.1

Details

• alias — Creates an interchangeable name for an existing type. File-scope only. Supports primitives, structs, enums, arrays, maps, and pointers. Can be prefixed with private.
• while — Identical to as_long_as, both produce the same token. User's choice which to use.
• else — Identical to otherwise, both produce the same token. User's choice which to use.

Scope

• Add all three to the KEYWORDS array in completion.ts
• Add hover doc entries in the DOCS object in hover.ts
• alias should also be recognized by the symbol scanner in symbols.ts for go-to-definition

[SchoolyB]

Implemented in e4a61d9. Correction to the scope: two of the three keywords were already done.

• while — already in the KEYWORDS array with a hover doc entry.
• else — already in the KEYWORDS array with a hover doc entry.
• alias — genuinely missing. This was the only real gap.

So the work was alias only: added to KEYWORDS, given a hover doc covering the file-scope-only rule, private prefix, compile-time erasure, transitivity, and what can and cannot be aliased.

Per the last bullet in the scope, alias is now also recognized by the symbol scanner. It scans as its own symbol kind, so go-to-definition resolves it and hover renders the aliased type (alias Meters = float → Type alias Meters → float). Handles the private alias form and trailing comments.

[SchoolyB]

The LSP half of this shipped earlier. The tree-sitter grammar also needed a change, now in grayscale-lang/tree-sitter-gray@0090150.

Two gaps: there was no alias rule at all (the only alias in the grammar was an import-alias field name), and otherwise_clause was hardcoded to 'otherwise'.

else was the worst defect found in this batch. It produced no ERROR node but silently mis-parsed: the keyword became a bare identifier expression statement and its block became an array_literal. Highlighting rendered else as a variable and its body as an array, with nothing to indicate a problem. Now parses as otherwise_clause.

The pinned grammar rev in zed-extension/extension.toml is updated, so Zed picks this up.

Verified against the 603 valid programs in the Grayscale integration suite: parse failures drop from 134 to 115 with zero regressions. The repository previously had no tests; test/corpus/features.txt now covers all seven features.