Add extension point for stylizing console output

[cottsay]

This change provides two noteworthy APIs:

• A set of "buckets" which call out specific categories of console output, like errors, warnings, package names, measurements, etc. This Style class can be imported in other colcon packages to annotate their console output.
• An extension point for performing specific text augmentation for console output in these buckets.

Some example use cases:

1. Colorizing (ANSI color escapes, etc)
2. Formatting (markdown, RST, simple character tables, etc)
3. Hyperlinks (HTTP, OSC 8, markdown, RST, etc)
4. Collapsible blocks (Jenkins, GitHub Actions, etc)

This PR contains a sort of "smoke test" implementation for ANSI color codes. I intend to remove that implementation prior to merging this PR. It is only there to demonstrate behavior, and I'm not interested in debating what colors are appropriate or what the "default" behavior should be. We can consider adding lightweight stylizers directly to colcon_core at a later date - this PR is about getting the framework in-place.

To see a demo of what the styles look like without performing actual operations, run python3 -m colcon_core.output_style.

For now, the interface only allows you to select a single output style extension. It may be desirable in the future to differentiate between "primary" extensions of which only a single one should be used, and "secondary" extensions which overlay the primary one.

[codecov]

Codecov Report

Attention: Patch coverage is 96.22642% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.50%. Comparing base (c8f5479) to head (df1e247).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
colcon_core/output_style/__init__.py	94.87%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #687      +/-   ##
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+ Coverage   87.29%   87.50%   +0.21%     
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  Files          68       70       +2     
  Lines        3951     4050      +99     
  Branches      760      783      +23     
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+ Hits         3449     3544      +95     
  Misses        396      396              
- Partials      106      110       +4     

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[nuclearsandwich]

• A set of "buckets" which call out specific categories of console output, like errors, warnings, package names, measurements, etc.

At first I thought that this implementation would only support style buckets defined in a central API. But I actually think that the way output style extensions work the style extension can decide which styles to implement. However, the call sites where styles are actually used must also only use styles exist in the provided output style, so we're realistically going to wind up with a centrally agreed upon list of styling options.

This differs from an after-the-fact decorator like Vim's syntax highlighting patterns or a markup, then style approach which allows for the style formatter to drop styles it doesn't know what to do with.

Do you anticipate any strife with having a limited set of well supported style options or is that an explicit goal of the feature?

[cottsay]

...the call sites where styles are actually used must also only use styles exist in the provided output style, so we're realistically going to wind up with a centrally agreed upon list of styling options.

This is a good point. I set up a default getter in 65a7364 that should allow call sites to use styles that aren't explicitly defined by the selected provider.

[nuclearsandwich]

Approval here is focused on the proposed OutputStyle extension API since the AnsiEscape example is included only for testing.