Support non-SemVer version numbers

[bric3]

Without thinking much I created a version with this scheme major.minor

pluginVerison = 0.1

changelog {
    version.set(properties("pluginVersion"))
    path.set("${rootProject.projectDir}/CHANGELOG.md")
    header.set(provider { "[${version.get()}] - ${date()}" })
    itemPrefix.set("-")
    keepUnreleasedSection.set(true)
    unreleasedTerm.set("[Unreleased]")
    groups.set(emptyList())
}

tasks.patchPluginXml {
        version.set(properties("pluginVersion"))
        // ...
        changeNotes.set(provider {
            changelog.run {
                getOrNull(properties("pluginVersion")) ?: getLatest()
            }.toHTML()
        })
    }

This generated a changelog version header like this :

## [0.2.0] - 2021-10-27
### Added
- stuff

The patchPluginXML task failed with this exception

* What went wrong:
Execution failed for task ':patchPluginXml'.
> Error while evaluating property 'changeNotes' of task ':patchPluginXml'
   > Failed to calculate the value of task ':patchPluginXml' property 'changeNotes'.
      > org.jetbrains.changelog.exceptions.HeaderParseException: Header '[0.2] - 2021-10-27' does not contain version number.

By looking how the code works I identified the issue here

gradle-changelog-plugin/src/main/kotlin/org/jetbrains/changelog/Changelog.kt

Lines 38 to 39 in f8f60be

	else -> split("""[^-+.0-9a-zA-Z]+""".toRegex()).firstOrNull(
	headerParserRegex::matches

By default if the header regex is semver

gradle-changelog-plugin/src/main/kotlin/org/jetbrains/changelog/ChangelogPluginExtension.kt

Lines 25 to 31 in f8f60be

	internal fun getHeaderParserRegex() = when (val value = headerParserRegex.orNull) {
	is Regex -> value
	is String -> value.toRegex()
	is Pattern -> value.toRegex()
	null -> ChangelogPluginConstants.SEM_VER_REGEX
	else -> throw IllegalArgumentException("Unsupported type of $value. Expected value types: Regex, String, Pattern.")
	}

gradle-changelog-plugin/src/main/kotlin/org/jetbrains/changelog/ChangelogPluginConstants.kt

Lines 38 to 40 in f8f60be

	val SEM_VER_REGEX =
	"""^((0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?)${'$'}""".toRegex() // ktlint-disable max-line-length
	}

I suggest to rephrase the string in HeaderParserException to mention a mismatch between the actual version and the headerParserRegex.