commands: add grep

[mbolivar-ampere]

This has been requested various times, but its lack never bothered me up until now, etc. Anyway, here it is.

Fixes: https://github.com/zephyrproject-rtos/west/issues/337

EXAMPLES
--------

To get "git grep foo" results from all cloned, active projects:

  west grep foo

To do the same with:

- git grep --untracked: west grep --untracked foo
- ripgrep:              west grep --tool ripgrep foo
- grep --recursive:     west grep --tool grep foo

To switch the default tool to:

- ripgrep:  west config grep.tool ripgrep
- grep:     west config grep.tool grep

GREP TOOLS
----------

This command runs a "grep tool" command in the top directory of each project.
Supported tools:

- git-grep (default)
- ripgrep
- grep

Set the "grep.tool" configuration option to change the default.

Use "--tool" to switch the tool from its default.

TOOL PATH
---------

Use --tool-path to override the path to the tool. For example:

  west grep --tool ripgrep --tool-path /my/special/ripgrep

Without --tool-path, the "grep.<TOOL>-path" configuration option
is checked next. For example, to set the default path to ripgrep:

  west config grep.ripgrep-path /my/special/ripgrep

If the option is not set, "west grep" searches for the tool as follows:

- git-grep: search for "git" (and run as "git grep")
- ripgrep: search for "rg", then "ripgrep"
- grep: search for "grep"

TOOL ARGUMENTS
--------------

The "grep.<TOOL>-args" configuration options, if set, contain arguments
that are always passed to the tool. The defaults for these are:

- git-grep: (none)
- ripgrep: (none)
- grep: "--recursive"

Command line options or arguments not recognized by "west grep" are
passed to the tool after that. This applies to --foo here, for example:

  west grep --foo --project=myproject

To force arguments to be given to the tool instead of west, put them
after a "--", like the --project and --tool-path options here:

  west grep -- --project=myproject --tool-path=mypath

Arguments before '--' that aren't recognized by west grep are still
passed to the grep tool.

To pass '--' to the grep tool, pass one for 'west grep' first.
For example, to search for '--foo' with grep, you can run:

  west grep --tool grep -- -- --foo

The first '--' separates west grep args from tool args. The second '--'
separates options from positional arguments in the 'grep' command line.

COLORS
------

By default, west will force the tool to print colored output as long
as the "color.ui" configuration option is true. If color.ui is false,
west forces the tool not to print colored output.

Since all supported tools have similar --color options, you can
override this behavior on the command line, for example with:

  west grep --color=never

To do this permanently, set "grep.color":

  west config grep.color never

[mbolivar-ampere]

@marc-hb for sure I'll be adding tests before merging. I'll update the --quiet and internal-reminder-when-adding-more-tools as well. Thanks for review -- other comments responded to.

[mbolivar-ampere]

@marc-hb please revisit

[mbolivar-ampere]

@marc-hb comments addressed and one additional bug in grep.<TOOL>-args handling found and fixed. Thanks for a great review!

[marc-hb]

and... it hangs forever :-)

Hotfix submitted in #681

Looks like personal configs are still "polluting" the test suite...