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 93.63057% with 10 lines in your changes missing coverage. Please review.

Project coverage is 87.51%. Comparing base (c8f5479) to head (7798cca).

Files with missing lines	Patch %	Lines
colcon_core/output_style/ansi_colors.py	89.47%	3 Missing and 3 partials ⚠️
colcon_core/output_style/__init__.py	94.44%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
+ Coverage   87.29%   87.51%   +0.22%     
==========================================
  Files          68       71       +3     
  Lines        3951     4101     +150     
  Branches      760      787      +27     
==========================================
+ Hits         3449     3589     +140     
- Misses        396      399       +3     
- Partials      106      113       +7     

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