Add accessibility package and isEnabled function

Add accessible themes

This commit leverages `cli` fork of `charmbracelet/glamour` with enhancement for configuring `chroma` formatter.

It includes simple tests that enabling `gh` accessible features causes codeblocks to be downsampled to base 16 ANSI colors.

This commit is v2 approach to testing for accessible colors and codeblock rendering using an in-house approach to identifying ANSI escape sequences and analyzing them for color depth.

After talking with @williammartin, this is likely going to be refactored a third time leveraging a module to help with parsing escape sequences from text.

This commit is focused on PR feedback around codeblock testing and simplifying related code:

1. Use of new `WithOptions(...TermRendererOption) TermRendererOption` to clean up `WithTheme()`

   The `glamour` TermRendererOption pattern has a limitation that users cannot compose multiple options without duplicating code or building one-off anonymous functions.

   However, this commit takes advantage of an enhancement in https://github.com/cli/glamour/pull/3 that allows users to leverage a helper to avoid building one-offs.

1. Use of new `leaanthony/go-ansi-parser` dependency for parsing ANSI escape sequences and display attributes

   In v1 of `markdown_test.go`, the codeblock tests were very simple, testing the result of output of markdown rendering against a string of ANSI escape sequences. The concern raised is that this was testing the result rather than behavior wanted.

   In v2 of `markdown_test.go`, the codeblock tests were refactored to use regex to extract and analyze ANSI escape sequences and display attributes. The concern raised was that this was a lot of logic to build and maintain and might benefit from a dependency that could do it.

   In v3 of `markdown_test.go`, a combination of v1 and v2 approaches for 1) testing that theme appropriate colors are used and 2) testing that ensures accessible display options are used when accessible experience is enabled

After discussing the changing nature of the tests with @jtmcg, this commit renames the test functions to make it apparent their purposes: testing we get the colors we want separate from checking for accessibility concerns.

In addition, there were a number of other improvements to be more idiomatic.

Ensure codeblocks use accessible colors

1. Introduce constant for `gh config` setting for accessible colors
2. Align existing constant for env var to match
3. Rename exported functions to distinguish use specifically for color
4. Minor enhancements to comments and tests

Following Go `x` convention seen across various projects, the former `accessibility` package has been relocated to communicate its experimental status more clearly to extension authors.

It has also been renamed to `color` to align with adopting Primer Style color role functionality.

Realign accessible color logic around new env var

Relates github/cli#830

Due to errors in PR, this commit updates GitHub Actions workflows to use a newer version of actions/setup-go capable of reading Go version from go.mod.

Major change with this commit is relocating experimental logic into the `x`
package to communicate clearly expectations to users.  Additionally, the
package in-line docs have been updated to state experimental nature more
clearly.

There are a few other odd cleanups:

- test assertion improvement
- better in-line comments

The experimental markdown colors shouldn't be used for any arbitrary purpose outside of `x/markdown` package.  This commit unexports several types and functions, leaving `x/markdown` functions as the main use cases.

Additionally, there are a few additional changes to improve understanding and reduce the code involved:

1. Use of "ANSIColorCode" has been replaced in favor of "glamourStyleColor" to explain their purpose
2. Replace "ANSIColorCode.String()" and "ANSIColor.StrPtr()" for "glamourStyleColor.Code()" to simplify and clarify use over types
3. Add comments to explain importance of enumeration order
4. Expand test verifying color codes

This isn't really needed beyond this package, so unexporting it to avoid unintended use cases.