meta: deduplicate source generation infrastructure (#10348)

* Move `Formatter` and `Error` to new crate: `cranelift-srcgen`

The `cranelift-codegen-meta` crate emits both Rust and ISLE source code
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meta: deduplicate source generation infrastructure (#10348)

* Move `Formatter` and `Error` to new crate: `cranelift-srcgen`

The `cranelift-codegen-meta` crate emits both Rust and ISLE source code
using a `Formatter` which may fail with an `Error`. The
`cranelift-assembler-x64-meta` crate uses a subset of this
functionality. To deduplicate efforts, this creates a new crate,
`cranelift-srcgen`, and moves only the `cranelift-codgen-meta`
functionality there (for now).

* Add notion of a formatted `Language`; append file locations

While developing `cranelift-assembler-x64`, it became clear that knowing
the source code location that _generated_ some code was extremely
useful. This change alters the `fmtln!` macro to append file locations
as comments to the generated code; this doesn't change all the
`fmt.line()` invocations but it's a start. To do this, we need to know
which language we are generating to emit the correct comment kind. This
led to adding a new `enum Language` to identify that.

* Rename `update_file` to `write`

* Use shared `Formatter` in `cranelift-assembler-x64-meta`

This change removes the duplicate implementation of `Formatter` in
`cranelift-assembler-x64-meta` and replaces it with the shared
`Formatter` in `cranelift-srcgen`.

* Add `Formatter::add_block`

While implementing all these changes, I noticed an opportunity: by
adding `Formatter::add_block` we can automatically adds braces and
indentation. This should be more safe (i.e., harder to forget to append
the ending `}`) and result in less verbose generator code (no need for
`fmtln!(...)` then `f.indent(|f| ...)`). This refactoring does touch a
large amount of code, though.

Now with `Formatter::add_block`, we can allow the `fmtln!` macro to
auto-format no-arg format strings (e.g., `fmtln!(f, "{x}")`), which
should be a bit more ergonomic. This does mean that a few locations that
weren't covered by `Formatter::add_block` need some extra escaping
(e.g., `{` -> `{{`).

* Avoid appending a file location comment to a Rust comment

* Make `cranelift-srcgen` publishable

This ensures the crate has a version and is tracked by
`cranelift-srcgen`.

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