Replace :! binding syntax with phase keywords and contextual defaults (#7479)

Implement the toolchain side of proposal #7254, removing the `:!`
binding
syntax for generic and template parameters in favor of the keywords
`generic`,
`template`, and `runtime` plus contextual defaults for phase.

For valid programs this is semantics-preserving: each binding resolves
to the
same phase, and produces the same SemIR, as it did under `:!`/`:`. The
parser
derives a binding's phase from its syntactic context plus any explicit
phase
keyword; new diagnostics and error recovery for misused keywords are
described
below.

Implementation details for each component:

- Lexer: remove the `:!` (`ColonExclaim`) token, move its virtual
parse-node
  budget onto `:`, and add the `generic` and `runtime` keywords.
- Parser: thread a `BindingContext` (`ExplicitParam`, `DeducedParam`, or
`CompileTimeEntityParam`) from declaration introducers down through
parameter
lists to each binding pattern, using a one-token lookahead to
distinguish a
name-qualifier parameter list from a declaration's own final list.
Parameters
of a compile-time entity (`class`, `interface`, `constraint`, `choice`,
`alias`, `export`, `namespace`) and deduced `[]` parameters default to
checked
generic; explicit function parameters and local bindings default to
runtime.
`HandleBindingPattern` resolves the phase from that context plus the
keyword: a
`generic` keyword needs no node of its own (the phase is carried by the
  binding's node kind), while a `runtime` keyword is preserved as a
`RuntimeBindingName` node so `check` can name it in a diagnostic. A
phase
keyword that is merely redundant with the contextual default is
diagnosed
  here, without invalidating the parse tree.
- Check: a phase keyword that is invalid for its context (for example
`runtime`
on a checked-generic parameter) is diagnosed here, and recovers by
building an
error binding that still introduces the name so that later uses of it do
not
  produce cascading errors.

The removed `:!` syntax is now rejected as an ordinary parse error.

The `form`/`:?`/`->?` ("extended types") portion of proposal #7254 is
left for a
separate change.

Assisted-by: Claude Code

toolchain/check/eval_inst.cpp

@@ -348,13 +348,13 @@ static auto TryFindValueInRewriteConstraints(
 
     // The LHS of a rewrite can be an arbitrary type. For example:
     // ```
-    //   T:! Z where C impls (Y(.Self) where .Y1 = {})
+    //   T: Z where C impls (Y(.Self) where .Y1 = {})
     // ```
     // The rewrite in the facet type will be `(C as Y(T)).Y1 = {}`.
     //
     // It can also be another ImplWitnessAccess. For example:
     // ```
-    //   T:! Z where .Z1 impls (Y where .Y1 = {})
+    //   T: Z where .Z1 impls (Y where .Y1 = {})
     // ```
     // The rewrite in the facet type will be `((T as Z).Z1 as Y).Y1 = {}`.
     //
@@ -371,7 +371,7 @@ static auto TryFindValueInRewriteConstraints(
     // TODO: Requiring the rewrite to be against `.Self` is actually
     // insufficient. For a facet like
     // ```
-    // T:! type where C impls (Z(.Self) where .Z1 = ())
+    // T: type where C impls (Z(.Self) where .Z1 = ())
     // ```
     // and an access like `C.(Z(T).Z1)`, the root of the access is `C`. If we
     // search `T` for a witness, we'd need the inner-most self facet to be `C`